Not really sure of the questions YOU are asking. Are you asking if it's legal to have somebody who is a CA (non LCA) occupy the right seat on an ordinary line flight operated under 121?

If so, yes, it's 100% completely legal.

My current employer has a tendency to do it when they are short staffed on FO's. Which quite frankly, I haven't seen since last summer. It may still go on, but not in my base. The only issue that ALPA has with it is the natural one. The more senior CA will be designated the PIC, of course. Also, it's not really specific to in the CBA, but I believe it's reserve CA's only.

I don't think ALPA really needs to address the issue as far as staffing the right seat goes. If the company is too stupid to pay TWO CA's to operate a flight, what is ALPA going to tell them? "Hey, hire more FO's, you need to staff better". Duh.

My prior employer's CBA had a different rule. Some of it makes perfect sense, some of it was rather silly. A newly qualified turbo prop CA could be required to sit in the right seat, but ONLY for the FIRST six months after his fed ride as CA. After that, he can't be called upon to occupy the FO's seat. A CA on turbo jet equipment could NEVER sit in the right seat, EVER. The logic being, he has put in his time (at that company), he can hold CA, why should he EVER be required to go back to the right seat and not be able to log PIC time unless he is displaced? And per that CBA, as soon as a turbo jet CA started FTD/CPT, he was not longer qualified to occupy the right seat on a line flight again.