As there seem to be no flights from ARN that would facilitate a connection of less than 16 hours, those should be eligible for a free transit hotel. If I remember correctly, flights in J with 8 hrs or more of layover time in IST or Y flights with 10 hours or more are eligible.

TK used to contract the Renaissance Polat pre new airport; now it seems to be the Pullman (which is fine, however, as pointed out above, it's an hour-long ride from the airport like virtually any hotel). Given the 1535 departure to SEA I'd still think one could get some decent "free stopover" value out of this. Also meals at the hotel are provided (dinner, breakfast). ​​​​​Y class pax get different hotels, most of which were non-international chain in my experience (had 10+ transit hotels in J in the last years).
YMMV with regards to the specific hotel, I once (pre new airport) ended up at some Arabic chain hotel (don't remember the name) which was...below average to put it mildly. All other stays were at the Polat Renaissance, the Pullman or the adjacent Mercure.