I don't see the point of return to FEL either, he's stated NHL is his dream and he's not getting any younger so a shuffle between Europe and NA doesn't make sense at his age. It's probably going to take him a year to adapt to NA hockey no matter what, return to Finland would just delay that adaptation time further.
The thing also is that he now clearly is/will be in contact with Granlund and must have firsthand information from Granlund how being (among) the best player(s) in FEL doesn't mean NHL-readiness, and dues need likely to be paid in AHL, and that spending an extra year in FEL probably isn't going to further the NHL dream in any other area than maybe having more time to build strenght. Which he can do playing in NA/AHL as well.
I think the deal is negotiated in a way that enables Keränen to return upon request (or I think that clause might come straight from the agreements between NHL and FEL?) in the case that Keränen is not able to show enough in AHL, some guys just never get their game working in NA hockey, isn't called up and it seems NHL career is unrealistic. At that point Keränen would probably return to HIFK. Can't imagine him giving up before giving it a real shot over full or almost full season.