Think you'll be doing a fair bit of crying on this project!
If this wall is going to continue to separate 2 dwellings you'll have to do a lot of work if you start playing with it. Acoustic separation is going to be rubbish if the floorboards run under the wall. Fire separation will also be rubbish.
Once you've mopped up the tears, think about a redesign rather than just correcting a bodge- sounds like the place is a blank canvas at the moment.
If this wall is going to continue to separate 2 dwellings you'll have to do a lot of work if you start playing with it. Acoustic separation is going to be rubbish if the floorboards run under the wall. Fire separation will also be rubbish.
Once you've mopped up the tears, think about a redesign rather than just correcting a bodge- sounds like the place is a blank canvas at the moment.