I do hope they let Mallow have a full episode with Shaymin, if they aren't letting her keep it(Be really stupid if they don't have Shaymin choose to stay honestly... if Lillie can own Magearna, why can't Mallow own a Shaymin? They're pretty much the same thing! Barely even been enough focus on it...)
Yeah, if Shaymin really is a temporary capture then they at least need to validate its existence during its run (even Poipole at least bonded with Ash and Pikachu throughout a lot of its tenure, poor Shaymin's got to share one filler episode).
I'm curious about the foreshadowing of how it will be released however. Shaymin turning into Sky Forme and flying away forever seems contrived when Shaymin is among few Pokemon that can change back very easily.
SM feels odd at this point, it doesn't feel remotely like it's nearing the end of its run, especially with the girls who still feel like they have lots of development to go through with their Pokemon.