You can go back. Mountain Pass is still active and is pretty big part of story.Someone else correct me if I'm wrong, butonce you go through the elevator from the Underdark to the Shadow Cursed Lands, Act 1 is done and you can't go back that way.
Just carry on doing everything in the Shadow Cursed Lands. No spoilers but if you travel up to the upper north end of Reithwin there is a Masoleum which has a really big quest for one of your companions
Need to ask a question:
Trying to finish things around the first area. I have found the owlbear cub but only after I have killed the three leaders of the goblins, but have left the camp hostile towards me. The walkthrough tends towards doing this before killing the goblins. I have spared the mother, is it still possible to recruit the cub from this scenario?
So you didn't really spare the mother. The goblins came and murdered it instead - thus the owlbear cub being at goblin camp. If it shows up past this point it'll be in your camp - long rest a couple of times. I'm guessing you'll have to clear out that goblin camp to move the story on. You could try knocking out owlbear cub if its hostile. Its a passive skill you can toggle on / off.