1) That Regenerationn "limit" you're speaking of is
debunked in this thread. Not only is the scene of Mercer being downed from getting his arms ripped off by Heller is treated as an inconsistent anti-feat that doesn't scale to his true level of Regenerationn at all (although the arm ripping was done multiple times, so even if we assume that this anti-feat still somehow applies, a single loss of both Mercer's arms would not be enough to take him down), but Mercer's Regenerationn scales to the Supreme Hunter (which is a viral being that is
designed from Mercer's very own DNA), who was able to regenerate from a puddle of their
ow biomass (whereas Mercer not only scales to the Supreme Hunter's feat, but he has to regenerate from far less biomass than the Supreme Hunter under worse circumstances).
This not only means that Mercer doesn't need biomass to regenerate (which also means that Mercer can regenerate even when low on biomass), but he only uses biomass to make the regenerative process quicker. This type of logic for Mercer's Regenerationn was not only accepted in that thread, but the changes was also applied to the profiles of Mercer and the rest of the Blacklight beings (along with the Supreme Hunter as well), where the previously existent limitation of needing biomass to regenerate is now nonexistent. Mercer only needs to consume biomass to heal quicker than usual (and even then, scaling from the Supreme Hunter's feat of being able to reform their hand from a puddle of their own biomass within seconds, that also means Mercer can just reform his entire body within several seconds without needing to consume as well), nothing more than that.
2) The skills Mercer gained from the "ordinary soldiers" are those who was trained to not only deal with terrifying viral outbreaks like the Redlight Virus, but they have been training for
40 years to prepare for the next outbreak after the first. The Blackwatch soldiers literally had 40 years of preparation and training on Mercer, and even they didn't do well against the Blacklight Virus like Mercer. And Mercer was able to consume around
thousands of them (and the scientists as well, but there doesn't seem to be as many of them being consumed as the soldiers are), with his militaristic combat and strategy skills
stacking up from each soldiers he consumed.
They may be humans (though if we're including the Blackwatch D-Code Super Soldiers as well, they are more than just humans), but the Blackwatch soldiers (especially the more experienced ones) have decades of training, experience, and preparation in their belts. That doesn't sound like your "ordinary" military soldiers to me (Heck, they were even portrayed to handle the infected and other various things better than the US army can in the Prototype 1 Web of Intrigue, where the US army soldiers looks like amateurs in comparison and has to listen to what the Blackwatch soldiers commands of them.
If even the US army soldiers has to step aside when Blackwatch soldiers gets involved, especially against the infected, then that's
evidence that Blackwatch soldiers are above your ordinary schmucks in competency). If even the Blackwatch soldiers aren't doing well against Mercer, then that speaks volumes of how skilled Mercer can be in combat.
So unless there is
explicit evidence that characters like Anima are better than thousands of people, I doubt Anima or Kaneki would have better fighting skills than Mercer. Also, you've forgotten to mention that Mercer has devastators as well and are assuming that Kaneki has the range advantage, which is definitely not the case as the Tendrils from one of Mercer's devastators was able to cover all of Time Square. Either way, fighting skills is irrelevant in this fight when Kaneki either gets absorbed by Mercer or gets infected by the Blacklight Virus.
3) Kaneki's inferior Regenerationn is not going to help him against Mercer, who is going to either consume Kaneki or infect him. Both of these are going to bypass Kaneki's Regenerationn pretty easily.
4) From what I've researched about the ROS virus, it does not have the feats or symptoms that puts it in anywhere near the level of infection of the Blacklight Virus or even the Redlight Virus (both of which could straight up kill their targets with a mortality rate of 99.999%). The ROS virus is about
certai amount of cells (Rc cells specifically) going out of control. The Blacklight Virus has feats of easily infecting and
reconstructing all the cells in the target's entire body. And that's just one of Blacklight Virus' less insane capabilities. And I doubt resistance to the ROS virus would give Kaneki resistance to diseases that can infect both living and inanimate objects alike, so I find no reasons for why Kaneki should be able to resist it, so he'll either become a mindless infected by Blacklight or he dies from the virus killing him the inside and out (thus, overpowering Kaneki's "Mid over time" Regenerationn). There's also the fact that attempts to develop cures against the Blacklight Virus has failed, which basically makes the Blacklight Virus even more incurable than the ROS virus (which was cured at the end of the series).
5) His attempts to try to squash Mercer through sheer size would either get him absorbed upon making physical contact with Mercer or get infected. And I've already made my points about Mercer's Regenerationn above (where Mercer scales to the Supreme Hunter's feat of reforming from a puddle of their own biomass within seconds), so he'll definitely survive that even if he somehow doesn't get his Absorption or infection out in time (which is unlikely as I'm pretty sure it's within Prototype 2 Mercer's character to use either of these two abilities right at the beginning of a fight. Either way, Kaneki is not getting past Mercer's Mid-High Regenerationn). And from what I can tell in Kaneki's profile, his AP is around baseline Low 7-C. Evolved form Mercer has baseline Low 7-C without further augmentations, so if he amps himself up with the Musclemass boost, he'll get 3x the Low 7-C baseline AP. I doubt Kaneki has much of an AP advantage over Prototype 2 Mercer with the Musclemass amp, but that doesn't matter much when Mercer's hax should be what defeats Kaneki easily.
Tell me why this is not a stomp again?
