1) Yes depending on the unit and the IV. If you get a bane on the unit's critical stat, spending 20k feathers might not be worth it, but for most other combinations there are enough buffs to mitigate any shortcomings. If you are still fairly new to the game, you don't need to worry about extreme min/maxing. Just try to get a team of 20+ level 40s so that you can do some of the chain battles.
2) You will eventually want 4-5 solid teams of characters, with redundancies in most roles, so just aim to build a diverse set of good units. Find one of the wikis online, and focus on units in the top 2-3 tiers.
Xander is a great unit. Camus, I no longer use as much. Between Cecilia and Reinhardt, my horse team can already handle most of his niche. I ended up replacing him with Brave Lyn. Horse teams are pretty flexible though. Just grab a mix of melee and ranged units from the top half of this list:
https://fireemblem.gamepress.gg/cavalry-tier-list. Then make sure you have the hone, fortify, and spur calvary c-skills on each.
3) All of the seals are worth getting IMO. Given how easy it is to level units right now, I'd go for it.
Here's my unit levelling strategy: Pick the unit you want to raise, and then pair them with 1-2 dancers and a healer. If you only have one dancer, include a high level character who can tank or kill units as needed. Pick an appropriate level of the training tower and refresh the opponents until you get a favourable mix of colours (if you are levelling a red melee, you probably want a bunch of green characters and no blue ranged characters for instance). When you get something that looks good, allow the unit you are levelling to get as many kills as possible. Preferably all of them. Your healer will keep their life up. Your dancer can either give the unit a second turn, or the healer a second heal. I find that picking a training tower level that is 3-5 levels higher than the character you are working on is a good optimization between getting more XP per kill, and still being able to kill things easily.
4) Masked Marth is basically a neutral Lucina with no skills besides her sword. She is pretty much always a bonus character in the Tempest Trials, and given how bonus characters now get boosted stats, she is very strong in the trials when you get her skilled up. Mine has 57 atk, 43 spd, 32 def, and 26 res in the trials with Fury 3.
5) I love using healers in the chain battles and tempest trials. They can keep your team alive through most or all of the rounds. I also use healers when leveling characters. I find them of less use in Arena, or during single battles, since those are typically over in 2-3 rounds.
