It was likely replaced with another used engine. No mechanic is gonna warranty that for 2 years, and I doubt they'd even warranty a new engine for 2 years unless they rebuilt it themselves. You'd have to take that up with the engine manufacturer / refurbisher or the person who sold it.
It's a Subaru, they blow through engines. A relative had a 2014 Forester and it had two engines replaced under warranty. This is apparently a known problem with a certain recent version of their 4 cyl. boxer engines.
For sure. If you want a Subaru that lasts forever, get the 3.6R - it's a bulletproof 6-cylinder paired with a heavy-duty transmission. They stopped making them about 2 or 3 years ago.