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Can a chapter simply wish to not serve the imperium and turn to chaos and not become all mutated? Or is that always an eventuality?

It's the Gods' decision who gets mutations and such. Perhaps your chapter has simply succeeded at being unremarkable and thus avoided the Dark Gods' attention? The OK Marines or the Maybe Later Marines?

If you don't worship the Gods then you don't get mutations.. You can be Chaos Renegades, but yes, USUALLY you get mutations, however some CSM warbands choose to cut out or remove any mutations they get, which would be interesting to see modelled ;D

There are examples of Chapters which have gone renegade without turning to Chaos worship. The Lamenters are one such example - they sided with Huron Blackheart whilst he was still a "mere" renegade but by the time Huron had converted to full-on Chaos (even this being out of necessity from having to flee into the Maelstrom), the Lamenters had returned into the Imperial fold.

Hell, if you got mutations just for railing against the will of the Imperium, the Space Wolves would probably be growing spikes and skulls from every orifice by now.

Service to Chaos eventually winds up with being exposed to the Warp, which leads directly (sometimes immediately) to mutation. If you're serving Chaos, you are, in some way, attracting the attention of those sentiences that rule Chaos, which eventually gets you gifts which are mutations.

The Soul-Drinkers Chapter even suggests that one does not even need to knowingly or willingly serve Chaos in order to gain these mutations.

I know of exactly one way to avoid mutation after becoming a servant of the ruinous powers. And that one turns all non psykers into bound golems, besides being hard to pull of:

The Iron Warriors do the cutting of mutations thing, but remember that some mutations are to invasive or inevitable to remove. You can't just chop of your entire skin or a mutation inside of or replacing your brain. And your new tantakles might just regrove on another place of your body, as the infection is spiritual, rather than merely biological.

However, that doesn't mean all chaos servants look like the pride of every freakshow. Many mutations aren't blatant and/or external. Otherwise cults and Alpha Legion opperatives would be much easier to root out. Just look at Abaddon, Huron and Kahrn. Without a doubt they are among the mightiest servants of Chaos, highly favored and giftet by their gods. And yet they don't look like the elephant man. So you might stand before a throughoutly corrupted enemy of the imperium and yet never know it until you cut him open and are assaulted by maggots where there should be blood and snakes where you expected guts.