This is how trademarks are supposed to work; "The Oatmeal" and "Oatmeal Studios" are similar and it's reasonable to think that consumers could confuse the two. At the very least, it's way more legitimate than the FunnyJunk crap.
Fourth option: Inman licenses his content to Oatmeal Studios, who then handle the greeting card side of the business and pay him a royalty.
(I have no idea of the relative turnover of these two businesses so I don't know if a licensing option is feasible.)
Then again, without direct knowledge of the principals, it is impossible to predict how this might play out.
It'll never last.