(3) Does the College of Arms grant crests only as an addition to existing arms? What if one were a U.S. armiger using non-honorary arms, would said crest then be an "honorary" crest used with "non-honorary" arms?
Yes, although now rarely because most English personal arms now already have crests. Up through the 17th century, there were many arms that didn’t have crests, and if the kings of arms deemed you worthy—however defined—they would grant a crest.
You have probably seen this before, but here is an example of a crest granted to existing arms: