LOL although DO residencies don't exist, in essence they do, they are still pretty much heavily biased towards DO, but NO you can't take the COMLEX as only DO students can sit for that.
Yeah I'm pretty sure that's because it's FM. Any competitive specialty program will expect you to rotate there, as they already expect this of their usual candidates. Obviously some programs can't audition enough people to feel wholly comfortable with how long their rank list is, so they invite non-rotators but in my experience non-rotators go on the end of the rank list 9/10 times.