For Paul's uses of "Brother(s) of the Lord." I'd like to take some time (not necessarily a huge word count), since there are known problems with "Who's Who" in Paul's Jerusalem. So, I would like to move carefully, return to this fresh tomorrow (my time), and also give you fair warning if you have an objection:
If there is no objection, then I will not argue for, but instead argue based upon the assumption that Simon, Peter and Cephas are all names of the same person and that James the Brother of the Lord and the James with whom Paul makes a deal years later are the same person (Galatians Chapters 1 and 2), and that this James is one of the Brothers of the Lord whom Paul depicts as having a wife (1 Corinthians 9: 5), as well as the James who gets his own appearance in 1 Corinthians 15:7. Other patterns of identification are possible, and Christian apologists may prefer other assignments of identity. That is my preferred pattern.
More later, and you can counterpropose a "Who's Who" if you'd like.
No, the next verse refers to another, larger group of people, the believers.
If your argument involves a whole complicated chain of reasoning to explain why the text says something other than what it says, then I will not find it convincing.