No.
Yes. What takes the coolant's place when it leaks? Air. The coolant reservoir is meant to compensate for *pressure*, not capacity/volume. Sometimes that's correlated, often it's not, particularly in situations like this.
No chance some rag got stuck in there while doing the engine swap? or something wild like that?
Compression tests aren't always a good test here. But if it's been bad then it probably was when you go it and it may be bad enough to show iteslf. Pull the plugs and see if one looks considerably different than the others. That can suggest coolant entering combustion chamber and burning off in there.
Those Phase I EJ22 heads are elementary to replace in the car. No need to even pull it. External head bolts! Easy. Pull the head, resurface (DIY sand it), install Subaru head gasket. Done. If you don't pull the engine it wouldn't be a big deal to just replace the one that's bad, not much wasted time if you ended up doing the other gasket later.
The main concern I'd have is that it sounds like you got this with existing head gasket issues and I'd wonder how badly it was overheated before you got it.