This question covers a lot of topics so this answer may be long.
First things first, as long as the post is readable there is no reason for downvoting or deleting based on the type of formatting alone. Readable means:
the text is generally legible
(some handwritings can be terrible but if it can reasonably connected to an intended meaning, it is okay)
Since it is possible to flag an answer for deletion and give the reason as "Link-only answer," I would say that they are definitely discouraged, if not outright disallowed.
A comment is definitely more appropriate. The same is true for one-line answers - brevity and directness are nice, but some explanation is appropriate in almost any case. Otherwise, a ...