Your points (1) and (2) seem like fully general counterarguments against any activity at all, other than the single most effective activity at any given time. I do agree with you that future suffering could potentially greatly outweigh present suffering, and I think it's very important to try to prevent future suffering of non-human animals. However, it seems that one of the best ways to do that is to encourage others to care more for the welfare of non-human animals, i.e. become veg*ans.

Perhaps more importantly, it makes sense from a psychological perspective to become a veg*an if you care about non-human animals. It seems that if I ate meat, cognitive dissonance would make it much harder for me to make an effort to prevent non-human suffering on a broader scale.

(4): Although I see no way to falsify this belief, I also don't see any reason to believe that it's true. Furthermore, it runs counter to my intuitions. Are profoundly mentally disabled humans incapable of "true" suffering?

(5): Humans and non-human animals evolved in the same way, so it strikes me as highly implausible that humans would be capable of suffering while all non-humans would lack this capacity.