Also check to see if your sideplate is bulging away from the frame under trigger pressure. Seen that happen resulting in an awful trigger pull.
dju

Well, it's been soaking in penetrating oil for days. Still stuck. Strangely I don't think it has been ground down because I can't get it to push back into its recess at all! Can I apply heat and see if I get it to budge that way?

Just go ahead and remove the pin, if the plunger still won't move, at least the hole will allow more penetrant into the small channel.

You may have to grind a very fine "starting" punch to get the pin to move. It is likely well stuck also.

A Little heat won't hurt, just till the residual penetrant bubbles or catches fire! BE CAREFUL.
Hold the piece with tongs and apply heat slowly or just put in a 350 degree oven(the part, not the solvent container!)

Then let it soak for more days.

The internal spring may have "gone south" for whatever reason, but mild-mod heat and continue soaking would be my plan at this point.

Guess I’ll update here as well, I fixed the ‘12 Luger, the spring in the channel was seized with something that looked like flash rust, but wasn’t. It’s years later now and I’ve never had another issue with that Luger. Got a GREAT deal on it too, and it’s a nice early one!

It is always great when someone lets us know how "it" turns out.
Even "if" over a year later! :)