For the seniority portion: I was thinking seniority would be more along the lines of TT rather than years, no need for national agreements or universal databases.
When switching to shop Y voluntarily (not talking of mergers here) TT can be given a multiplier; say if you are from a different shop but the union has a reciprocal policy then give a multiplier of for example 0.95, 0.85 if from a union shop with no reciprocal policy, and 0.6 for non-union time[or student, CFI, etc].
(The multiplier is to discourage folk from playing hopscotch just to abuse the system also to appease those already in company Y.)
So why would I tolerate some other guy with similar total flight time popping in just ahead of me in seniority? Partly because I'm not a petty childish douchebag but mostly because on a large scale this strengthens the unions' side across the industry while also rewarding honest companies. If company Y negotiates real sweet contracts and they needs some pilots, while company X does all sorts of horrible mean nasty stuff when it comes to wages, then some of those pilots can leave X and come on over to Y. At this stage Y is gaining proven experienced pilots and company X is loosing pilots so can't service all of its routes, which leaves room for Y to expand service and add yet more pilot jobs under a good contract and agreeable management.
Just win win all around from my angle.