How should one go about abbreviating (a)synchronous? Is the most correct form (a)sync. or (a)synch.?
The common and my preferred way of abbreviating synchronous is sync, rather than synch, partly because it makes more sense to pronounce it as sink than sinch and the shorter abbreviation is simpler and clearer. Also to avoid any confusion with the slang word cinch (pronounced like sinch).
For similar reasons and consistency, I'd recommend async over asynch.