Are these worth bothering with? If so, how to use? how to design, how to build, etc.?
Probably. If you have to fight in mountains, mountaineers might be handy yes.
I mean, I'm not sure what else you were expecting really, but there you go.
So, what I'm seeing is: They are ~25% better for assault, as well as being particularly better for defending non-flat terrain, and for a cost increase of 25 to 27% in materials and 38% more Infantry Equipment.
The 38% more Infantry Equipment means: more small arms, more mortars, more grenades, etc. The 25 to 27% more in materials, I'm not sure. Better uniforms? Better bivvy gear? Trained in facilities that are slightly better?
The funny thing is: in the 8-wide incarnation they occur at the outset (at least for Germany, I guess Austria's templates are more beefy) they don't seem particularly useful..
It does annoy me you waste a research slot researching Mtn/Mar/Par, which let's be honest are a luxury and it unlocks a 4 Batallion division so you need to splurge 50/60 army experience making them worthwhile. They should start at 18 width like most starting infantry divisions are.
From an historical standpoint, I think having to commit through two loops of "research" (or more) to have really effective marine/para/specops is a good thing.