If Sam's medallion won't let him shapeshift properly, couldn't he still go to fullform by just leaving it at home?
Depends on how it's broken. It could be that the medallion just gave him the Unturned legacy spell again, rather than the controllable version it's supposed to create (or just didn't break the old spell at all), while still bonding to him so he couldn't use another. In that case taking it off wouldn't work, and even giving it up permanently might not improve anything (but would definitely make a real fix impossible). Hanging onto it and hoping he can fix it someday is his best option.
Or it could be a case of Mama's Baby, Papa's Maybe...he's half gryphon, his mother is human. The simplest explanation is his father isn't really his father at all.
Jim may have unknowingly completed the reason for the eldest son visiting a far-away location: Finding a sphinx. (Though Jim is not in fact the eldest son, just the oldest of his generation to have inherited the Finn "curse")
Genius Bonus: Michelle is a winged Grecian sphinx which are traditionally female. Her father was a wingless Egyptian sphinx which are traditionally male, It's not just a nod to both legends, it's Bizarre Sexual Dimorphism.
Michelle's Magic Pants don't seem to apply to her shirt... which may be a nod to to images of the classical grecian sphinx, who is invariably depicted topless.
Male-to-Female Universal Adaptor hasn't appeared in the comic yet but it's practically mandated by all of the interspecies couples, many of whom could only have sex in human or midform because otherwise their intimate parts wouldn't even be compatible. It shifts from Fridge Logic to Fridge Brilliance when you realize that one reason poor Anthony is so put out by his transformation into a male harpy because it probably means that he, like most birds, doesn't even have a penis.
No one has even suggested Sam might be a case of Mama's Baby, Papa's Maybe, which implies that similar "Medallion Failures" aren't entirely unknown even if they're not common.