I know it isn't ideal but our house has plaster walls with wire mess and tearing out the shower tiles and the tiled shower floor will be a massive undertaking (I know, we already did it the "full way" in another bathroom. That said, unless it is feasible... I guess that is what we will do. But just in case, here are my questions...
If the existing tiles are solid, can I apply a roll on waterproof membrane like Redgard to the entire shower including floor and then tile over?
I have heard of some quick hack ways of doing this, personally I wouldn't reccommend them. The proper way is of course what you already know. Lots of work - no doubt. Additionally, when doing a quick hack, you may run into other troubles, such as plumbing/drainage, tile too high, etc. Just my thoughts.
I'm sure you can google, "retile over tile" and a lot of quick suggestions will pop up.
Sorry I couldn't be of more help to you. It's good you have questions, hopefully they will be answered. Stay tuned, and get some more advice/recommendations from the pro's that frequent this forum before you move forward. Just a suggestion. Thanks.