re: "I would advise against relying on a 15 year old tire as a "spare"..."
Never said the spare was 15 YO.
If fact, the spare is newer than the 2 tires that are on the axle, but it doesn't match and the tread pattern is different.
The plan is to buy 2 new tires, retain the current spare as a spare and cleanup all 3 rims.