In case it helps anyone, the developer @rahilqf I engaged completed the project including translation of custom MyBB user fields into Discourse custom user fields (which depends on having the multi-select-user-field plugin installed), MyBB post prefixes translated into Discourse tags, and stripping out of nearly all redundant MyCode tags from MyBB post text.
His customized import script version is shared here - Custom MyBB to Discourse Import Script.
He also successfully merged in a Yahoo Groups Mbox archive, using the standard Yahoo import script - though with this attachments seemed not to translate. In my case that was not a major problem.