Have you looked at what Media Composer does with Mix & Match for different frame rates and de-interlacing? If that quality is satisfactory, then the process is pretty straightforward. If not, then external conversion might be needed.
I used SubCap to subtitle my film. Although it doesn’t have the best quality, it passes. You can export out as EBU-N19 and DS Caption. I use the master caption window rather than the individual text entry box.
Just keep in mind that if you’re using DMF to create media, then the files don’t have a project type associated with them. Background transcode does not have this issue.
Do a mixdown within Avid in an RGB 709 project choosing ProRes 444 then export with “same as source”. Exporting and setting the codec is now up to the whims of the QuickTime engine.
If you use AAF “Edit Protocol” with 6.5 and earlier there was no need to transcode to export an AAF. In v7 I don’t need to transcode to create full version of AAF as long as it’s a “link to” and not embedded. Not sure why you need to transcode.
Are you doing an embedded AAF or a “link to” AAF? I make AAF “link to” all the time with a mix of codecs and there is no need to consolidation or transcode. Just as a test, if you are doing “link to”, try checking the “AAF Protocol” to be active and try again.