I would add
4: Magus is altering the normal process of the Dewitchery "Diamond". All bets are off from the get-go. We don't know how much of the Diamond's normal functioning Magus is changing. Enough to make it explode suggests a fair amount.
KISS Principle means Magus is not going to care if he gives his spells to Elliot or if he gets Elliot's spells. He wants his body back. I would guess the rest is negotiable. Magus might not even get any ability to "pass on the curse" (this assumes that Magus is coming out as the "permanently cursed duplicate" which may not be accurate, but if we consider the last time Elliot touched the diamond, Magus would be the cursed duplicate. What "pass on the curse" ability would Magus even get? The ability to turn people into permanent spirit-realm shadows? Fuse people with their dimensional doppelgangers? Fuse people with Elliot?
Magus claims to be a wizard. If he's a Wizard by EGS Prime standards, he will probably get Elliot's spells. If Elliot has a buried Wizard affinity or picks it up from having been fused with Magus, he *could* get Magus' spell list, but I don't know what he gets just from exposure to the Jewel. He got Awakened last time, so if he has a buried Wizard Affinity, the diamond would likely bring that out.
There's a wildcard that occurred to me. Magus was originally involved with both Pandora and Sirleck. And Pandora is a very intelligent, predictive thinker. Suppose she set Magus up with Sirleck and pretended not to care just to create this moment. We don't know what magical advice she gave to Magus. Perhaps she finessed her magical advice in order to finesse the results against Sirleck. Using Magus as a pawn against Sirleck seems a very "immortal" way of thinking.