1. Valiants are some of the best heroes in the game. Leon and especially Thea are weak units overall (Leon is usable tho). For instance, Grand Ranger Kane is a monster in both PvE and PvP if you use him in a ranger trigger squad. Meanwhile, Lucille offers a two turn immunity for the team(inquisitor) or somebody(archbishop). Therefore, it is generally safe to max them out.
2. All buildings have their uses and I'll leave it to you on what you would need as you go along the game.
3. Use them to farm the normal difficulty story quests. It's fine to use them. I would advise to especially keep Luna(the 4* ranger) for her attack trigger aura because she is relatively easy to limit break. Also, I advise you to keep your phoenix hatchlings and only use them when you are upgrading 5* heroes because it takes a lot to max out 5* heroes and hatchlings offer a constant 20% during merging.
4. There are different playstyles in this game which is what I like but the best composition is the ranger trigger squad. However, I'm telling you that it requires so much resources so you shouldn't immediately aim to build a ranger squad. Some viable PvP squads are:
a. A mystic stall team which aims to utilize the mystics' MAG stats to inflict damage while they are protected by your healers and guardians. This is an okay start for a PvP team because they do not demand everything maxed out unlike a ranger team. Of course, investing in them also makes it scale so this squad is generally the safest route on a PvP team. However, this kind of composition kind of sucks in PvE.
b. SDD(single damage dealer) squad which is great for raids. This literally means you only use 1 unit as a damage aura and the other 3 or 4 are aura slaves which lets your attacker refresh or attack indefinitely. (You can search SDD teams in yt to see what they do and what heroes can you use.) This isn't the best PvP team but it is viable.
c.The ranger trigger team, as I've mentioned earlier , utilizes 4 rangers and have them trigger attacks for as long as a ranger lands a crit. Note that this team, for it to truly shine, needs all rangers heavily invested(like at least 60% crit chance). It is the best composition for PvE and PvP because it is the most resource intensive squad.
d. There is also a healer trigger squad, similar to the ranger trigger squad. This is also a resource-intensive composition, however.
You can search some vids on youtube about the squads I've mentioned for you to see what each composition can do.