Basically, our reasoning behind it was as follows.
We feel that we have a duty to make sure that all of those we were responsible for in the Lexicon know that they have a home here, should they choose to take us up on it. We do this out of loyalty to those individuals, not to the Lex. We just felt it was the right thing to do.
We are going to press on, I think we've done an excellent job at establishing our own identity so far, and we'll continue to do so. We just kinda felt that this is one of the things that needs to be done, though, in order to gain "closure" and to bridge the gap between what once was, and what we have created here. As time goes on, that section of the Constitution will become all but irrelevant. But serves a purpose for the time being, while many are still confused as to what exactly happened.
As for your second question, I can't think of a circumstance which would preclude a former senator from being allowed to sign, though that doesnt mean one doesnt exist. There would probably have to be some major conflict of interest, though I dont know what form that would take. It's a very unlikely scenario.