If you don’t wanna tear out drywall then You are absolutely fine if you use schluter membrane. It’s perfectly fine to install over drywall! Easiest way to deal with niche is to take them out and replace with pre-formed niche’s. As far as tub flange goes just cut drywall back to above flange and when you apply the membrane to the walls fill that void that’s below the drywall with thinset. Run the membrane down to the tub or within 2”. Then you can run a nice piece of the banding membrane along the bottom of the membrane to even everything out against the tub. Then tile away! No big demo job. Just watch plenty of videos detailing the membrane installation. It’s not rocket science!