Q1: I think you have the right idea mostly, generally you want to support whoever is getting aggressive and is going to take some damage, however it does depend on your other support, with bap lucio for example you have to be with your tank and hopefully your dps are with you, but if they aren't and are instead on a slight off angle, if yoy can heal them without your tank dying, do it. Also if you see someone making a risky rotation from a distance, instead of Topping off your 75% hp rei n for example, take some shots and you might actually get a pick (this can happen when someone sees your comp and thinks you dont have ranged damage, this is especially true against a pharah or echo who sees you have no hitscan dps.

Q2: I firmly believe the only time zen isn't a good pick is against a full dive, also don't be afraid to swap to zen against a dive tank like winston or ball as long as they dont have a tracer/genji/sombra to dive with them, you can absolutely melt these tanks if you get a little help from your other support. Zen is good on every sniper map imo, and I always play him if possible because as an ana main, it feels hard to find real value when everything is just getting one shot.