And why can't he see his opponent?
It is said right there on the scan you linked.
1. Popo's speed was too great for Goku to keep track of. Goku asks why didn't he see anything.
2. Popo says Goku was only looking with his eyes and that he needs to clear his soul, sharpen his spirit and feel the slightest movements in the atmosphere (this is not only extrasensory perception, it is also enhanced senses).
And that's how Popo taught Goku how to keep track of faster opponents. That has nothing to do with speed. The only speed part here is Popo blitzing Goku or demonstrating his own speed while specifically telling Goku that he needs to move faster than lightning.
Guru's power is a mystery yeah, though the fact that he considers being able to defend the dragon balls in his youth from Frieza shows he was at least higher than Porunga.
No that statement is taken out of context and incorrectly translated.
The conversation was how Guru was about to die in a few days and that Krillin needed to hurry up before that. And that Guru was not able to move from there and not even Nail would be able to protect him if Freeza arrives. Nothing about being able to protect himself or his balls from Freeza in his prime.
Also forgot to mention Piccolo in this as well, given he's still part of Dragon Clan but shows his chi to be much more better than his magic.
He is different case given how he is unable to make dragon balls and the dragon balls disappeared when Kami merged with him, meaning he isn't your regular dragon clan Namekian and clearly an abnormality given how powerful he was as compared to the rest of his race. That's why Goku asked if he can un-fuse again. Kami clearly possessed much more magical prowess given the scenario while Piccolo only had next to nothing. And that was the whole reason why Goku went looking for a new dragon clan Namekian. That analogy is unrelated to Kami who was so much more magically inclined as to be able to create the dragon balls which only the dragon-clan members can do.
Your comment itself points out why this feat is gag and isn't meant to be taken seriously.
Even the manga pays direct homage to the beloved Japanese "Rabbit on the Moon" folklore for children where rabbits live on the Moon making mochi. Toriyama also says that Monster Carrot and his crew survived the explosion of the Moon and are still floating in space. So I guess they have Moon level durability now and those humans can survive in the vacuum of space, but Goku and Vegeta can't? This has gone from validating Roshi's outlier to validating every other blatant inconsistency/outlier/gag in DB which would ultimately invalidate a massive plot point of Freeza saga and RoF arc.
Also, being able to tell when the power pole should stop extending/retracting wouldn't be possible if its speed was sooooo much greater than Goku's, who couldn't even see a Popo who was only lightning fast. (We not that on Monster Carrot's profile because we don't assume he is a regular human given his...you know...appearance and such, but honestly I am indifferent about giving him the ability to survive in space and it doesn't really matter here.)
He has a point, it would be more consistent within the scaling if we assume Kami is 5-C regularly than what would be "6-C with chi, 5-C with magic" if this revision is knocked down.
It would have made perfect sense for Kami to be 5-C regularly if we didn't treat Roshi's feat an outlier, because powerscaling. But as long as Roshi's feat itself is invalidated, we have no reason to assume as such.
I mean he literally had the lottery in hand, it seems rather absurd to think he wasn't going to do it or it would only fetch "likely / possibly higher"
I will agree with you here. While I get that an empty statement by a guy who called himself "limitless/omnipotent" depending on the translations and constantly hyped himself as someone who is unbeatable even though he knew of Kami's existence, is hard to believe without him backing it up. But I personally don't think King Piccolo would have said something like that to the whole world if he wasn't capable of that, when he was just going to demonstrate it in a little while. I mean, talk about the embarrassment he would have felt if it didn't happen the way he described to the whole world after talking so big. I think we can get it calculated how much energy is needed to destroy 1/43rd of the world's land
in the same way he destroyed this city as an example.