Any inquiry is going to be limited to the SPFL's role so, if any shenanigans happened behind closed doors and didn't involve any SPFL staff, we may never find out why Dundee changed their vote.
There is no solution that will keep Rangers onside. Hearts MIGHT be placated by an increased parachute payment but Rangers will only settle for voiding or, possibly, by the removal of Doncaster.
From what we've seen so far, there is no such thing as a reject vote in company law. Discussions and persuasion can continue for 28 days to force/nurse a resolution over the line.