Can you trade waiver position?

Waiver position can't be traded like draft pick but a claimed player can be traded immediately (but still following the session transactions rule) would be my interpretation

Can a GM switch from PH GM to Player GM or vice versa when losing a player to GM another team or disciplinary action?

It has been allowed in the past, but only to the departing player's position.

Do Co-GMs fall under the category of able to skip waivers and going straight to FA?

At this time, no. There is no precedent for this as Co-GMs have never been codified before this season.

Do hiatus players need to fill out the offseason survey?

No. They default to staying with the team and keeping their current build though.

If a GM steps down in the middle of the season but remains a player, what are they allowed to change?

Position changes are free for PH GMs. Build changes (pitching type/bonus and batting type) require MiLR credits to change in-season.

Are retired players allowed to change positions or builds when they unretire?

If they would have had the opportunity to make changes to their positions or builds during the period they were retired, they are allowed to make changes (i.e., if there was a period in the offseason they could have freely changed). If they retired at the start of the season and then un-retired a few sessions later, then no, they are not entitled to change any aspect of their player for free and would need to use MiLR credits to do so.