Sure doesn't sound much like a battery problem to me either since you did take a voltmeter reading.
If it were me I would try the fan movement again by either pushing in some slack between pulleys, or by tightening the belt. If that still fails and you don't want to remove the starter then I'd take the sparkplugs out and try turning the engine again.
Personally I've never heard of a starter seizing, so I doubt it would be that.
If the engine is stuck the longer you wait the worse it will get. Shouldn't take much to get it freed up after just 3 months. Of course that is just my opinion.
