Should I Save Quartz For Merlin?

Merlin is very useful but not at all necessary. Think of him as an unfair power-up or cheat code that the game offers you to make the game easier. It makes the game much easier to play but with you can still complete the game with 3 stars and a friend support with clever strategising and good ol' RNG. But he does have incredible synergy with a lot of servants and team compositions, and having him will allow you to do the Memerlin wombo-combo team.

You have Jalter which is arguably the best servant to pair with Merlin, so I don't see why you shouldn't save for Merlin, unless you have another servant that you like over him.

If you want easy peasy lemon squishy and boring game, save for Merlin. Merlin can greatly boost your buster servants like JAlter and Carmilla.
As for summer servant, you may take a look at Archuria Archer, she's considered a broken Arts servant, tho again it depends on what kind of team that you want to build. If you want to go for full Buster Meme, then save for Merlin obv.