better mpg, most of the electrics the same, however derv is much slower than the 4.0l though a chip improves things abit. Hot start can be a problem to as can cracked heads.
All in all a good choice though.
I think you need another Garage, that said it also sounds like they are trying to cover all the bases, and don't really know what the true fault is.
All the parts though taken one by one are quite easy to do yourself. I really would try and get a second opinion though.
it depends on what has gone wrong. usually you can get in with the drivers door key. Also if the battery get low the system will un-superlock the system so that you can get in if the battery goes flat.
It super locks the remote not via the door key.