Join Date: Aug 2011
Location: Barcelona, London, on a plane
Programs: IB+ Gold, TK E+, Hyatt Globalist, Marriott LTP, IHG Diamond
Posts: 14,185
First of all, it can't hurt to see whether SPG or the hotel will make an exception and refund your prepaid booking. If not, then indeed check in and mattress run it...
As for the second hotel, the only 100% safe way to get credit is to stay with a different chain.