Hi Folks, I am new to the forum

and pretty new at working with ceramic tile. I have however some experience in putting down ceramic floor tile, but I am no pro and had help w/ the project.
Project I am working on is a bathroom vanity back-splash. Attached is a very poor picture of my current bathroom back-splash that don't show the whole area. I will attempt tonight to take a better pic of the whole area. I want to tile across the back of the vanity counter top and the sides as well with a combination of small Mosaic glass and stone w/ mess backing followed by some small pieces of tile trim to edge it out. Here are few questions I seek advice on.
1. Removal of old tile. Should I remove the old tile and the drywall it's attached to and replace the drywall? Is it possible to remove the tile and keep the wall intact however I fear the job of removing old thinset.
2. What are the recommended wall prep for the tile. Is it ok to tile directly to the drywall? Should I put a waterproof sealant on first? backerboard necessary?
3. Space between the counter top and beginning of tile. SHould I put some sort of spacer here so I can either grout or caulk this area for waterproofing. Whats your advice?
4. What type of grout should I use sanded or non-sanded? The 12x12 mosaic has small 1x1 natural stone mixed in with it as well. Also is it ok to use a sealer on the mosaic glass tile or will it stain it or blemish it?
I am sure I'll a few more odds and ends questions. I want to thank you in advance for you reading this and taking the time to reply. Thanks much!
Chuck