LA has fewer routes, yes, but all of them can still be used to get to NY with only a few "extra" wagons needed. So, initially it seems to be a bottleneck, but losing e.g., LA to Phoenix is not a game-ender for you.
You are on the right track, though, in identifying the endpoints which have the fewest number of routes available to them. Vancouver, for instance, only has 3 routes available (2 if playing with 2 or 3 players), so getting any route into Vancouver would be a priority if your destinations went there, even if it is not your preferred route. You can't complete your ticket if you can't get to both endpoints, so getting into those endpoints may be the highest priority. If, however, you have Denver to Chicago, both of which have several routes in, getting into the endpoints early is less important as you can more easily adapt to the cards you are dealt.
As for adjacent tickets, getting from NY to Montreal is a priority, of course, but not necessarily via the 3-blue route. You can go through Boston (which will actually get you a longer train, but take 2 turns), or you may even go through Toronto or SSM - it all depends on your other tickets and at what point in the game you drew the 2 adjacent tickets. That said, if I have a ticket to (or near) Atlanta, I'll almost always grab Nashville-Atlanta as one of my first moves. Same with Houston-NOLA. They're low cost and can easily connect some otherwise-disparate routes.
All of this is dependent on the train cards you draw, of course. Your initial plan may be to go LA-Phoenix-Denver-and on to NY, but if you're getting a lot of black and yellow cards but no white cards, you may instead go LA-El Paso-OKC-and on to NY. If you're drawing a lot of blue cards, then the NY-Montreal route is a priority; if you're drawing a lof of red and yellow cards, then it's less of a priority and NY-Boston is.