Is there a way to preorder just the game? I'm not interested in merch.

What merch do you mean? I don't think the game comes with any pre-order bonuses.

By merch I meant the collectors edition. I just want the base game.

There is no collectors edition for PC. Only consoles get that and its exclusive to the NISA store. On PC you can only buy the base game. https://www.nisamerica.com/ys-x/products
If you mean the Deluxe version that has some DLC such as costumes and items, then that should be available here on day 1.

Yes. I don't see a reason not to. The build is already uploaded on GOG. Also Durante in a recent interview said that it's more easier now to upload builds on gog and epic
https://www.rpgsite.net/interview/16422-ys-x-nordics-interview-durante-pc-port-local-coop-hdr-exclusive-features-ph3-nis-america

Why don't they offer just a base game too? A game with two DLCs and an artbook is too much for me.