Are flags encouraged that indicate that a comment is actually an
answer to the problem?
Sure, but it would be faster to simply
@ the commenter and tell them to make their comment an answer. That's what we would do; it is all we can do, per my answer to your second question.
If we read comments that actually propose a solution to the problem,
should we flag them as "not a comment" for a moderator to reclassify
as an answer?
We do not have the ability to switch comments to answers; we can only switch answers to comments (and I've done that a fair number of times). All we can do in that case is simply
@ the commenter to ask them to add their comment as an answer. Veterans of Stack Exchange typically oblige, but people new to the culture typically don't and we are stuck.