Wood paneling

I have a duplex rental unit that I rent out, and have a lady who is moving out because of the noise from the tenants on the other side. It has plaster walls. What I thought was, when I get in that apartment maybe I could blow insulation in the wall cavity between the two units, to dampen the noise. So that the next person who moves in, has more quiet. How hard is this process? I've seen a Bob Vila video where they drill a 2" hole here and there, top and bottom every 16" and then blow stuff in. I assume you rent the blower unit. Has anybody done this before, and how hard and expensive was it?

Ceiling in bay window is cracking, looks like right along the bead on the header outside corner (I think that's what to call it). I chipped off a bit along that line and it looks like they didn't tape over the bead, but there also seems to be some give in the ceiling drywall, like it's not fastened to the joists really well. I searched for nails/screws, and can only find what I've marked with red in the last picture. So from that, I think I can tell where the joists would be - I've done lots of knocking, but am having a hard time hearing any difference in sound. I know I can put a small nail through to confirm, but thought it wouldn't hurt to ask here before I go putting holes all over the place. Can I be pretty sure that where I've marked in blue in the last picture is where the joists are? Any others that are typically in a bay window? Feel free to draw in.