Is there a good way to saw this down, or is the best alternative to use cross battens x 2 to reach the old ceiling's level? Unfortunately, I can't redo the wall since my son is sleeping on the other side and the house isn't particularly large.
Haha, have you been in my house and done some demolition? I have the exact same layer of interior ceiling on a similar base of boards in most of our catalog house from '68 
In your case, however, I can't think of anything simpler than lowering the new interior ceiling. A slightly more labor-intensive solution that maximizes ceiling height is of course to drywall the wall and cut the panel and tretex level with the upper stud on the wall, and then re-drywall with drywall according to the new ceiling height. Alternatively, cut level with the current drywall and put full-height renovation drywall over this.
In your case, however, I can't think of anything simpler than lowering the new interior ceiling. A slightly more labor-intensive solution that maximizes ceiling height is of course to drywall the wall and cut the panel and tretex level with the upper stud on the wall, and then re-drywall with drywall according to the new ceiling height. Alternatively, cut level with the current drywall and put full-height renovation drywall over this.
There are actually 38 years between our houses. It looks like the room I'm messing with has had 3 different ceiling coverings so far. Papp, tretex, and morapanel.
Considering that the goal was to achieve a uniform ceiling and sloped ceiling (the latter was uniform with the wall before) that also fits the style of the house, going with gypsum and all seems a bit too much. I'm already replacing the insulation and installing a vapor barrier...
Anyway, attempting to use the multi-tool seems like the best option. The question is how fast you can go before the blade gets too hot?
It would have been fun with OSB behind the gypsum, but I happened to tear up the floor as well...
Considering that the goal was to achieve a uniform ceiling and sloped ceiling (the latter was uniform with the wall before) that also fits the style of the house, going with gypsum and all seems a bit too much. I'm already replacing the insulation and installing a vapor barrier...
Anyway, attempting to use the multi-tool seems like the best option. The question is how fast you can go before the blade gets too hot?
It would have been fun with OSB behind the gypsum, but I happened to tear up the floor as well...