Proper approach (in my humble opinion): Have your trees as own entities rather than mixintos. Then get Jelly, override jelly:index:trees with your own JSON containing definitions for your own trees.
That way, not only can you get only your trees to spawn, but also in whatever configuration you please*
Never happened to me. The error likely happens when the entity is created, as that seems to be the time when mixintos are applied. So a broken mixinto won’t throw an error until whatever you’ve mixed into is loaded for the first time.
* While Jelly does work as one would expect, there’s still one or two things that are a bit odd when you want to do completely customised stuff. If you simply want to replace trees in the rough same way the game does it (or would do it with override), Jelly is your man.