I would like to change the rake on my 1.5/1-1/8” fork by 2mm, but I can't actually change the fork, so I'm thinking about a Wolftooth GeoShift headset, They come in 1º & 2º options and for short and long options (90-114mm & 115-140mm).

My head tube is 280mm long (yes, 280mm or 11").

Which GeoShift headset should I use (long/short/1º/2º)?

I’m guessing the long 1º GeoShift headset would provide about 2.5mm on a fork with an A-C of 400mm with a 280mm head tube..

Am I close?

Maybe I'm quibbling, but I think it's good to keep in mind that you are not changing the rake, you're only changing the effective head tube angle. Yes that changes the trail, as the subject line says, but in the opposite direction to what your first sentence says ("I would like to change the rake").

Adding rake at a given HTA lowers the trail, but a headset can't do that, the rake* stays the same. Lowering the HTA via this headset, at a given rake, raises the trail. Are you OK with that?
If lowering the trail is the goal, then steepen the HTA.

I'm not familiar with those headsets, but I assume they can go either way?

* Unless you mean "rake" as the motorcycle designers use it, which is what we call HTA. To those guys, rake is a feature of the frame, not the fork. To them, forks have offset, not rake. Confusing for sure, but let's stick to the bicycle definition, since this is a bicycle forum, OK?

Yes, they can go either way. I understand they don’t change the rake, but most numbers bandied about are around rake. The fork in question has a rake of 57mm and is way too twitchy. I have a fork with a rake of 55mm which is perfect, so I’m trying to end up with a similar result (N=many…).

You ought to divulge the current HTA, rake, tire size and when the bike feels "twitchy".

It is unlikely that you can feel the difference of 2mm of rake, so I would suggest giving the specs of the good bike as well.