I respectfully disagree with your position. I'd hate to see any of my "bread crumb" comments converted to an answer. Ideally, following Jorge Castro advice makes the most sense to me. Convert these tiny partial answers to a full fledged good answer. This may require any number of efforts on your part from further research to editing for clarity and comprehensiveness.
Regarding your suggestions.
There is an easy way to politely suggest/remind the asker to mark an answer as accepted. It's called a comment. I've done this and I don't find it to be an onerous task.
creating a new flag function to route a comment into the answers where it belongs (perhaps), giving the user a choice... While at times, a number of comments can be combined to create a valid answer (I have done this as well and again did not find the task to be painful.), I have yet to see a single comment that qualified as a valid answer. I believe this approach would result in more poor answers.
New users are already provided with the information. You cannot force reading and comprehension on those who are unwilling or unable to do so. Allowing users to post comments on day one would increase the effort required to maintain the site, resulting in more account related work (warnings, suspensions, terminations?) and increase the number of useless comments that need to be cleaned up.
Note that these are still my opinions based on 2 years, 9 months months of service here.