IME, depends on the type and the conditions. IME, pre-merger UA certificates (same as today's 1016 type) were sometimes limited to an itin. with 1 person and sometimes not. That definitely was the case with % off, which makes some sense. With $ off, it doesn't seem to make too much sense as it would be a straight amount off anyway, whether for 1 or 5 people. Not that difficult, IMO to basically split the discount among several people.

The TCV certs (that are a form of payment) can be used on multi-person itineraries, and as many as one per person - they need to be applied to a specific person though. So if you have two certs and one person traveling, you can only use one, but if you have two people, you can apply one to each. IME, if you have one cert on a multiple person itin, it will be issued as 1 PNR, but you receive separate eTicket emails - one for each person which is different than on a regular two-person itin.

And yes, with even one flight on another carrier, none of the other payment options usually show (gift card, TCV, travel bank, etc.). TCVs definitely have the UA/UAX only restriction, but travel bank doesn't. I flew to BKK with 1 segment on ANA over a year ago now and couldn't get the travelbank option to appear, even though there is no UA-metal only restriction. It took me 6 calls or so and a great agent to finally get it used - she was able to look at the terms, agree that there was no restriction with travelbank and got it done, calling me back about 2 hours later with everything resolved. It was a large amount I was trying to use (through a gift registry), otherwise there was no way I would have bothered.