1- No author (anonymous) threads/messages
If there are Red warning with PMs from newbies, I don't think we need to see anonymous PMs. Don't contact someone else if you want to stay anonymously.
2- Multi author threads/messages:
Some of complex messages is hard for working on it. İf we can give permission to someone for write on it, it will be easy. (like google spreadsheets options)
It will create many drama when accounts banned because of plagiarism. Perm-banned 'victims' will open ban appeals and say "Oh, it's not a plagiarism because we work in a group, with a multi-author threads/ messages".
And moderators have to spend time to investigate who initially created documents, who edited lastly and who stole contents before spreading out. It is not a good idea. We have limited staff resources on the forum and they have to be free of unnecessary stuffs. As you pointed out, there are other platforms to do it, just stick with those platforms.