Without taking pictures I spent at least 90 minutes at Casa Batllo. I used the audio guide; if you just want to look, not listen, that might speed you up a bit--photography aside, of course. Taking advantage of the early time slot is a very good idea, because Casa Batllo really packs people in, and you'd have a very difficult time getting pictures during the regular hours except on the roof.
I didn't have time to get to Montserrat on my 2016 trip, but I suspect you can make this happen. See what Enric says. I know that the train departs from the rail station at Placa d'Espanya, which you can reach easily by Metro. On the day I passed through there, a uniformed rail employee was helping people buy tickets at the vending machines. I think it's a combo deal that covers more than just the train, and you have a choice to make about the last leg. Review the material in your guide book so you know your preference. Be sure you know what you are buying; it is probably a round-trip ticket, so don't buy another one at Montserrat to get back to Barcelona!