[@thewizardguy]

1) There are certain conditions that must be met before he can wield a person's weapon. He must either defeat them in battle, form a strong bond with them, or be gifted the weapon. 
2) Yes, he can use those abilities. HOWEVER, his attacks won't be as strong and will require a lot more energy than usual. In the case of using a weapon requiring black blood, his body will adapt to meet those requirements, but it will use fake black blood. This means that it will have the same offensive capabilities, but he can't use it defensively, since it'll still be normal blood for him.
3) I've yet to determine an exact number, for there are quite a few characters Michael has encountered. However, if I need to place a limit, I will do so.
4) Yes, it can. So long as one of the criteria mentioned in the first answer is met, Michael can wield the weapon. 

If there is something that you don't believe is right, I can always make modifications.