2018 Valentine's Challenge quests

You're gonna want at least one servant at max level, yes. It's a brutal fight against multiple servants in a single wave.

Yes you can use command spells/SQ to revive. Yes you can complete it with 1-3 star servants. I did it with Arash, Shakespeare, Hans, and double Spartacus. You will want your main damage dealer(s) to be at max level/ascension. Each enemy has roughly 150K HP and there are 14 enemies of all 7 classes.

Gamepress did their own guide on how to complete the challenge quest. https://grandorder.gamepress.gg/revival-valentines-2018-lite-challenge-quest-guide

Yes, but it is better to only use CS and fully ascended servants. I beat the challenge with 3 command seals, Mash, MHXA, George and Friend Merlin but you can do it with way cheaper teams but they need to be fully ascended to last the fight.