1. Why are you re-exporting? No, that's not what you do. You use the MERGE CLIPS command. This will make a NEW clip with the audio and video files merged. Typically you have a slate to sync to, or timecode is jam sync'd so the camera and audio recorder have the same timecode.
2. When you merge the clip, all the audio channels will be present.
3. Sorta. If you get PluralEyes that will automatically sync everything...but will do so by making a sequence with the video and audio. So a sequence of sync'd audio...not a clip. And if you use the sequence as a source, you need to make sure to use the OVERWRITE WITH SEQUENCE CONTENT mapped to the F keys, otherwise FCP will make those nested clips, and nested clips are not a good thing in this case.
An alternative to exporting:
Once you have synced everything merge/link the desired audio to the video (3 &4 boom audio/1& 2 can or cannot be linked)
select the clip/linked audio and drag to a new bin
You should see a dialog box that says creating master clip
clip in new bin will now only reference the newly linked audio.
Drag all clips into a sequence and you will no longer have to have camera audio(if that is what you desire) on the timeline and you will always have a reference in the original sync sequence.