Hi mdc_decker and welcome to the Mattermost forums!
Mattermost supports 16383 characters per post, at least on recent versions, but yes, this will definitely require some additional and better handling. Long posts could be split up (threaded, ideally) automatically, f.ex.
The import functionality is designed for initial migration of existing messaging solutions towards Mattermost, there’s not much handling with regards to merging existing user accounts, so the import just refuses to use them. It’s always hard to tell what information should be used then for the accounts if they have matching identifiers (and what is the matching identifier?), so not sure if there’s an easy solution for that.
Can you describe the usecase a bit more? Are you trying to import an additional (new?) team into your already existing mattermost installation where some of the team members are also already available in this Mattermost setup? I think there should be a way around this by removing the user creation command for the overlapping users and replacing their posts with the real Mattermost user IDs, the import should work just fine then, but it will require a bit of manual tuning of the export file.
There’s no way to clean the import jobs right now using mmctl or the API, sorry.