tub surround - advice needed
I recently purchased an old row house. The existing tub and surround were in pretty bad shape and there appeared to be mold so I decided to demo the existing tub and surround. Demo was easy as both the tub and the surround were made of plastic. Behind the tub I discovered greenboard, which now has some large pieces of paper missing when it came off with the adhesive. The wall where the plumbing fixtures go and the back wall are both framed with 2x4s. The long wall is plaster on brick and the greenboard has been glued to the plaster. There is no visible vapor barrier; it seems it was just greenboard and a plastic surround. I did not have to remove much wall material to remove the old tub or install the new tub.
I can remove the material from the two studded walls but demoing the long wall that has been glued to the plaster is going to be very difficult. This is also the only bath in the house and I have a lot of pressure on me to get it working.
I went to a big box home improvement store today and purchased enough Kerdi and/or Redgard to do the job. The Kerdi seems like the only "approved" solution for the mess I have but the Redgard seems easier to apply and a shorter lead time before I can begin to tile.
My preferences, in order, are...
1. Apply the RedGard, let it cure, and start tiling
2. Apply the Kerdi, let it cure, and start tiling
3 .take the two walls down to the studs, install a vapor barrier, install durock, and start tiling. I have no idea what to do with that long wall in this scenario.
I would appreciate any and all advice. This bathroom and the entire house will be taken down to the studs for a remodel in the next 18 - 36 months so I need a good solution but I'm willing to live with good enough for right now.