1. Yes. There are presets for guitars that aren’t amp sims. With eq, compression, reverb, et al already applied. Or there are eq that are general use (hi pass and low pass filters) that can be used on any instrument. Also, that snare preset may sound great on a guitar…who knows?
2. Yes. Things that sound great by themselves may sound like poop when “mixed”. That is the essence of mixing in general. It’s like making a puzzle. All the pieces need to fit. The pieces may sound great by themselves…like the drums and bass…but when combined…it loses its goodness. There may be frequencies building up and masking each other so neither sounds good together…separately they kick ass…together…poop.

The thing is, EZmix won’t know what is working and what isn’t. Presets are limited in that way. You may get lucky and stumble across the right preset for the fix, but maybe not. That’s when I reach for a manual fix (eq, compressor, whatever) and dig in and fix. Presets may get you in that ballpark but manual adjustments will get you on the field.

EZmix can be useful even with already-amped guitars, especially for adding polish and consistency to your mixes. Since EZdrummer 2 and EZbass already sound well mixed, you may not need it for every track. I’d try the demos or presets first to see if they add something valuable. It’s a bit like Letter Boxed: having the right tools helps, but the real difference comes from how you use them!