A 2psi difference at different operating points and different time of day isn't much of a concern, since the system volume will vary by temperature, and the pressure will also vary by that much between pump-on and pump-off states. But if it continues to drop even with the fill-valve fully off, you have a leak somewhere.
If the new expansion tank was sized correctly for your full system volume the t & p valve shouldn't continue to spit with the fill valve fully off. If the expansion tank undersized for the system it could still test fine at 12psi, but could still see overpressure at the system high-temp.
Bleeding/defective autofill valves that slowly bring the system pressure toward your street water pressure are a common culprit for chronic over-pressure issues. Auto-fill valves aren't really all that useful, and seem to cause as many problems as they solve. Filling/pressurizing the system manually then turning the fill valve fully off and checking it at the beginning of the heating season, or whenever the system complains is generally OK. If the pressure drops low enough you'll hear the boiler complaining with some sizzle rumble & bang due to the micro-boil on the heat exchanger plates becoming more macro than micro, and it'll be pretty loud long before there's any hazard condition or damage.
If that boiler is 40+ years old (and it looks like it might be), the economics of replacing it with something right-sized and higher efficiency might be favorable. Even if it's in pretty good shape it's probably not getting better than 75% efficiency, and it could easily be under 65%. A high-efficiency boiler would get at least 90%, but you'd not be able to vent it into the old flue, and the water heater would also have to go, since by itself it doesn't put out enough heat to keep the flue drafting, and would have guaranteed flue condensation issue plus potential back-drafting issues.