If you count this episode as in the same continuity as TSFS, then TSFS goes back to not making any sense.
We know the dead body is definitely needed for the ritual transfering the katra from the katra holder to the afterlife. The movie novelization implies that Spock, being half-human, didn't think he could transfer his katra to a holder (at the time he wrote his will), which is why his will specified that his body not be returned to Vulcan if he died in service of Starfleet, which is why Kirk launched Spock's dead body into space instead of following Starfleet regulation regarding the return of a dead Vulcan's body to the planet Vulcan.
If Spock had already transfered his katra to someone in TOS, then that totally unravels that explanation because if he had already done it then he would know he could do it. If he knew he could do it, then he would have willed his body to be returned to Vulcan, then it wouldn't have been launched into space, then it wouldn't have been regenerated on Genesis, then Spock would have stayed dead.
So if anyone's got another explanation as to why Spock's body was lauched into space after the katra transfer to McCoy that doesn't contradict "Return to Tomorrow", please post it. Otherwise, I'm just gonna delete RtT from my personal Star Trek continuity to allow TSFS to make sense.
I don't think it unravels at all. If you take into account that Spock's body wasn't actually dead in that episode, despite what Henoch might have believed. Spock (and Chapel) also had the experienced assistance of Sargon and Thalassa for all the musical-chairs mind transfers that eventually occurred.
This very experience might have been what prompted Spock to decide on his will as stated. If he found the experience particularly difficult he may have decided to spare anyone else the burden. His change of heart during the Genesis crisis may have been something akin to his Galileo 7 gamble.