ok, I'll start off with the "he was in Ripper Mode" argument, because it's the one argument that simply doesn't have a lick of validity and thus can be immediately removed
There are some very blatant problems here, first of, we can see from the very videos you posted that the eyes differ greatly,
here is the glint during the Armstrong bossfight,
here is Ripper Mode, you can see that they are two COMPLETELY DIFFERENT things, in the first one, it's just his pupil that lights up and the iris stay black, while the complete opposite is true for Ripper Mode, where the iris is completely red while the pupil is perfectly black. This alone would be proof of the two things being completely different, but there's more; the "red pupil" (that's what I'll call it) is consistently shown to appear in situations where we know for a fact Raiden hasn't gone Ripper Mode, but is merely showing determination, such as
here, where we are shown that Raiden stays in base throughout the whole ordeal; hell, this phenomenon isn't even exclusive to Raiden, and happens to multiple bosses, for instance
Mistral and, you know,
ARMSTRONG HIMSELF. All of this is proof that (and I'm sorry if I'm too direct) the red glint shown before the last part of the fight doesn't mean jackshit.
On top of this, let's also add that Raiden isn't in Ripper Mode during the bossfight in-game, which was the case in the only other case in which Raiden canonically went in Ripper Mode. Now yes, you can make up all the bullshit, gameplay-related excuses you want for why this is, but then, if they really wanted to show Raiden go in RM but didn't want to give him infinite fuel cells like with Monsoon, why the **** did they not just show him be in RM in the cutscenes/QTEs? Not only is Raiden blatantly in base form in the cutscenes, you can see him act in a way that is completely antithetical to how he would behave if he were in Ripper Mode, he straight up stands there and listens to Armstrong's last words, remember what happened against Monsoon? Compare it to
this and tell me if they are even remotely alike.
There is also the matter of Jack the Ripper and Ripper Mode, which are actually 2 separate things: Jack the Ripper is the violent persona Raiden has which basically leads him to be bloodlusted and violent, whereas Ripper Mode is a physical change, where his eyes turn wholly red, he manifests an aura and becomes significantly faster and stronger; the former is part of the latter, in the sense that every time Raiden goes Ripper Mode, he also goes Jack the Ripper, but the contrary is not true (or at least, doesn't have a basis in canon), so when Raiden goes Jack the Ripper he doesn't necessarily go Ripper Mode; this is perfectly shown in the very first scene of his transformation, wheree he first goes Jack the Ripper (
his pupils and pupils only turn red and he says "Jack is back") and only after Dok turns off his pain inhibitors he goes Ripper Mode (
he emits the aura and has red iris), this proves that it's 100% possible for Raiden to turn into Jack the Ripper without going Ripper Mode.
Essentially, there is quite literally no valid argument to be made to support the idea that Raiden went Ripper Mode in his last fight with Armstrong. No, "he would do it in-character" is not a valid argument by itself, since it's contraddicted by everything we are show.
Now, onto the Murasama argument.
Moot, we know from the fight with Sam that the Murasama doesn't actually effect Armstrong by itself, he can deflect it while being casual without suffering any damage whatsoever; while yes, Sam is probably quite a bit weaker than Raiden at this point, Armstrong is also very much suppressed, while he's straight up bloodlusted against Raiden with the Murasama, and we know that Armstrong's suppression effects his dura significantly.
This stuff is only true or relevant if you don't look at the entire fight, because, for one, you can clearly see that all the scratches on Armstrong's arms are caused by that very QTE, as they are not present in the scene right before and his arms are
completely fine, showing that whatever permanent damage Armstrong suffered only happened after his punches were slashed with BM. What happened with the heart is that Raiden concentrated himself on one spot, scratching it over and over again until he was able to create a small hole in which he could shove his hand. The fact that Raiden can casually crush Armstrong's nanomachine heart is already, by itself, proof that he physically scales to them, but what makes the "he's just chipping away at his nanomachines" argument completely moot is that 1) Raiden can punch him to the ground almost immediately after obtaining the Murasama, making it an impossibility for him to have chipped away at the nanomachines beforehand 2) the various scratches are only visible on his forearms, raiden can straight up punch him in the face and gut and hurt him significantly.
The idea of Armstrong's nanomachines getting chipped away or stuff like that is also completely unsubstantiated and Dok doesn't even mention it as a possibility when talking about ways in which Raiden could beat Armstrong, the scratches could very well just be an instance of Raiden's strength having grown to they point where he can surpass the nanomachines' hardening. it's also worth noting that the Murasama still bounces off of Armstrong's fist, which means that the nanomachines are still doing their job and countering the HF blade's effects, otherwise Raiden would have cut his foreharm in half.
This one isn't even the only case of Raiden's strangth growing through his fight with Armstrong: Armstrong starts off suppressed and suppresses himself less and less as the fight goes, he's at peak suppression in Phase 1 (when Raiden still has his sword), he is slightly less suppressed during Phase 2 (when Raiden fights him h2h and gets no-sold) and at the very end of Phase 2 he's straight up bloodlusted. Now, in Phase 1, with this level of suppression, Raiden gets
completely squashed by Armstrong and is completely overpowered by him; now, right after he gets his sword snapped, Raiden manages to successfully block one of Armstrong's punches and
straight up overpowers and overwhelms him up until he gets more serious, at which point he simply gets no-sold. This is a clear cut case of Raiden growing stronger during a fight.
Another case is the Monsoon one. We all know that Monsoon fights a RM Raiden, and while yes, Monsoon is weaker than Raiden in this case, he still puts up a good fight, fast forward no more than a few in-game minutes, Raiden fights Ai-piloted copies of Mistral and Monsoon, since there are no statements about it, and we don't see Raiden go RM, we have to assume he went into this fight in base, and he manages to beat both Mistral and Monsoon. Now, while Dok does state that "
the AI will not perform up to the organic original in many respects. Such things as speed, agility, accuracy, how it responds to damage", it's also true that these bodies are perfect copies of the real thing physically speaking, as proven by Dok's statements such as "
backups are built for custom-made frames like Mistral and the others. They can be used for spare parts in case of damage, or if the original body is completely destroyed... The brain can be transplanted over for a quick recovery" and "
Well, bear in mind that these spares retain all (of) the original body's abilities". Given how we know that Ripper Mode is a significant amp (hell, OP argues that it's big enough to let Raiden kick the shit out of people who no-sold him in base) we either say that Raiden managed to beat an enemy who can basically one-tap and no-sell him, or we say that he grew in strength in the meantime; while yes, the copies are slower and less skilled than their real counterparts, they can still use all of their abilities, such as the Phosphorous Smoke Grenades, which basically blind Raiden, and the detachment, during which Raiden can't canonically do anything but
wait for him to be forced to reassemble due to lack of oxygen, stuff like this would make it nigh impossible for Raiden to beat an opponent THIS drastically stronger than himself, and besides, it's just common sense to say that the protag is at least comparable strength-wise to the opponent he beats. On top of this, Raiden fights Sundowner (again, in his base because nothing leads us to believe he went RM) who could be argued to be even stronger than Monsoon due to being the leader of the WODs.
Now, I'm not saying that Monsoon is equal to Raiden's Ripper Mode, what I'm saying is that he put just as much of a fight against RM Raiden as Sundowner did against base Raiden.
To add to all of this there's a quote from Raiden during his fight with Sam, in which he says "
His fighting style uses a good amount of blunt strikes and throws too... but sooner or later, that won't be enough to stop me", which heavily, HEAVILY implies that Raiden is growing throughout the fight; this could be either Skill or AP, but given how he talks about "stopping", it's quite probable that he's talking about physical stats.
Now, frankly, I'm interested in proving that Raiden's strength grows more than defending/proving how he did it; now it's RPL, which I don't think is accurate, because we saw with Monsoon and Sundowner that Raiden just grew stronger in-between battles, this leads me to believe it is a very quick form of AP Accelerated Development, which also goes along well with his already existing and irrefutable Skill AD; this being said, I'm open to proposals