Normally I'm a DIY man. Six years ago I expanded our tiny bathroom, moved walls and all fixtures. Learned about Kerdi, Ditra, tile, the whole bit. Couldn't put tub tight to wall so made a 5" lip/shelf along back edge for shampoo, razor etc.
Last year we gutted our main bathroom and started to re-do it. Wanted a shelf behind the tub as well, as it gives an extra 5" width when standing in the tub.
By the time I got the orange fuzzy all installed we decided that since our neighbour is a "tile man", and offered to do the tile gratis we should take him up on it. His job looked great but due to other things, it was six months before we got to try the shower.
When I designed the shelf, I made sure that the Ditra on the shelf was level, then put a gentle slope on the tile using the mortar so the water would drain off. Our neighbour sloped this one to the REAR, so the shampoo bottles wouldn't slide off. After every shower there is a puddle along the back of the shelf~!

Now I have spare tile,(and grout) and with the design I can install new properly sloped tile over the old and have it look original. The question is, what should I use for mortar, latex mod, regular mortar or epoxy? I can rough up the old tile with a grinder, but I don't want to break it out as it is under the adjoining tile

Well you get what you pay for.

In your shelf description you mentioned there was Ditra on the underlayment. Did you mean Kerdi?

If your in Ontario I would suggest you get the Kiesel primer and a bag of Servolight thinset to bond tile over tile. It's available from Tilemaster http://www.holtenimpex.com/TM/lh.html

Probably, I can never remember which is which. A few years ago you couldn't get either locally (Bancroft), now I can even order the preformed corners...Great stuff.
As far as getting what you pay for, this guy has done some stone and block work for wife and I in past. Does lovely work and we have always been happy. He seems to feel that since he deer hunts out behind the barn that he owes us something besides venison .
I do get fed up with paying someone to do a job that I could do better myself. This normally happens when I hire a drywaller or framer. Then I step back and realize that I could have done it myself 20 years ago.....

Six months before you could use the shower? Man, I'll bet you were stinking by then.

There are several brands of thinset that will bond tile to tile. I don't know what the cost of the products are that RC mentioned, but it doesn't sound like you will need very much.

However, if you see that you will have to build it up to more than 1/4", you might want to do it in more than one layer. Build it up to 1/4" the first day, let it dry, then build up more if needed or install the tile with your desired slope. If the shelf is only 4-5" deep, you should need no more than 1/8" slope (probably even less) to keep the water off.