Now tackling the original topic of OP
First of all the surface of Saviour's exterior is permeable,
literally described as if it were incorporeal by Nero, which allows Saviour to absorb stuff freely. The notion that Sanctus has to manually activate said ability makes no sense when by default absorption properties are possessed by Saviour's skin, because as the scan says that's what allows him to move freely in and out of Saviour and same principle allows Kyrie to remain trapped in it, used as bait and switch constantly by Sanctus as shown in video.
Second, Nero is there stuck in Saviour's hand for long time even after Sanctus took Yamato, if Nero is truly helpless against absorption, Sanctus could have him digested at anytime, but he didn't.
Instead he chose stab Yamato into Devil Bringer.
Now some people say that getting Yamato spalshed with blood is somehow serving an external purpose which is why Sanctus did it. What the said purpose is, is not something justified by opposition here at all.
Let's steelman this point for arguements sake, what does that actually change for Yamato nulling Nero? Nothing! Like we have direct visuals of Nero getting power nulled. We don't need statements at that point.
It's a unequivocal fact as shown in the game that Yamato was used to negate Nero. His Devil Bringer lost it's glow and powers.
Another point I saw braught up was that Nero's blood was needed for Statue. Which makes no sense, Nero is already getting absorbed and assimilated there in body and soul. What purpose is removing extra blood doing?
If anything, it actually explicitly proves resistance for Nero. If drawing blood was explicit requirement for making Nero absorbable, then that means Nero was incapable of being absorbed prior to fullfilling this blood prerequisite. That automatically fullfills resistance requirements LMFAO.
And all of this is still extra compared to graphic showcase of power null done by stabbing Yamato into Nero. Which still stands unrefuted.
Another point that is being braught is this Yamato plus Nero's blood required for final stages of plan. And when asked what final stage of plan is, the answer given to us by OP is Hellgate opening.
At that point the only thing left to say is that OP and Co. has extremely limited knowledge of the game and it's lore.
The final stage of plan has always been using Saviour to play a false god and bring impress the people into worshipping the Order for world domination and recreation.
Saviour was made for literally that purpose.
Unfortunately when Saviour was created, it was inoperable despite it's immense power. Agnus had almost give up hope on it.
Up untill this point Yamato existed as broken pieces, being experimented on by Agnus. Which is what allowed Agnus to create false HellGates, which facilitated creation of Saviour in the first place by providing large amount of demons.
But Yamato still remains broken, unable of being fixed, and the plan to open true HellGate is a pipedream.
Then they find out Dante and Sparda sword exist, both together capable of supplying large power to Saviour and controll it successfully. Nevermind that any attempt to capture or outsmart Dante into Saviour would have been futile for obvious reasons, admitted by Agnus himself. And Yamato is still broken, which means it's still not a priority to open the main Hellgate.
It's only chance luck that Nero encounters Agnus and Yamato and ends up fixing the blade, proving he is descendant of Sparda. From then on the variables changed.
Opening of Main HellGate is actually possible now, and Nero serves as excellent substitute to replace Dante.
All that I have said is covered in novel, here.
All in all this blood connection thing with Yamato for Hellgate is completely nil. It has nothing to do with it.
Because ever since the Order found broken Yamato, they always wished to unlock main Hellgate, well before Nero ever came into picture. His blood wasn't prerequisite back then not is it now.
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TLDR
Visuals of power null are blatant, and have no satisfactory counter.
There's no connection between Hellgate and Nero's blood with Yamato.
Blood being needed as a prerequisite to have Nero get absorbed actually proves he had resistance originally, otherwise Saviour cannot absorb him normally.
Saviour doesn't need actively absorb anything, his exterior by itself is permeable in nature, pretty much any demonic entity can get absorbed in it upon contact.