Just to be clear, you've been posting designs that are well-crafted enough that the only thing I CAN say is a flavor quibble. Well done.
I'd be happy to accept that setting, if you will, for Sick Pet - Carrier is just a creature type where you need a "reason" to use it imo.
This is just me being stubborn but I would always prefer to see
Magic's Gnomes be artifact creatures. That's one of the unique universal flavor quirks of this game - apart from the set that's full of it-doesn't-counts, Legends, and
Quarum Trench Gnomes - and I'd like to keep it that way just because I want the M:tG-D&D barrier to be thick as possible.
Boggart Pot-Thief is great, but could it untap the artifact?