The modified thinset with additives is very strong and won't add much thickness. Back butter the rocks too to ensure good bonding. You DO need to remove all loose mortar, etc. and power wash or scrub it so it has a clean surface to bond. Since it is near water a light acid wash would help to ensure it is clean with no crud messing up the bond.

Mine is 30' inside dimensions in the long part and then 25' across the shorter/diagonal area: http://ths.gardenweb.com/forums/load/pools/msg0510133424402.html?15 Here is a link that might be useful: My Pool

I can't speak for pre-cast concrete, but we have a pool with natural stone coping which obviously can't have a rounded edge. It is free form, too, and the chiseled edge looks great, so I don't think the aesthetics of straight vs round should concern you. As for leaning your neck on it, I would say the amount of time people use the spa vs the amount of time people spend hanging out by the pool at the fire pit pales by comparison, and the way it will feel wouldn't be my first consideration but rather the way it will look.

Using your $10,000 auto cover as a protective tarp is not a good idea. Better find another solution for that. Also, the cover rails are attached to the coping most likely and will need to be removed and re-installed which essentially is a full auto-cover installation at that point. Be sure to budget time and money for this. Unless you have a unique coping forming system for concrete, or some very careful and diligent masons setting your coping stone, this will result in damage to the tile band. If you are pouring a new deck and coping at this stage then you are assuming risk for this - there are no two ways to say it. Good luck - hopefully you can get your project completed without any issues.