Nah - that's the inner joint of the CV axle. Swapping axles will change the symptoms and verify. It could be the bearing, they're supposed to be installed with a hub tamer, not a press. If they damaged it by using a press then the bearings and hub will need replaced. Your symptoms are classic inner CV joint issues though, the bearings can cause varying noises around turns but usually aren't so throttle dependent, that's classic inner CV stuff.
Get a new OEM axle or used OEM and reboot it and never think about it again
If the seized axle thing happens on the other side - just take the knuckle/axle off the car, reboot, and reinstall as an assembly, no need to remove the axle from the knuckle. I've even done them without removing the axle from the knuckle but that's probably not most people's cup of tea and is a better fit for the newer 2000+ style outer boots that last forever and don't need replaced every time an axle comes out.
speaking of which - i think you can use a 2000-2004 legacy axle on your car, i used to use them all the time on 95-99 EJ's when I used to work on those older subaru's. they have better outer boots and they're newer. i don't think a FWD imp is any different so that might be a good option.