I drive a manual and had the same questions myself when thinking of remote starters. Glad I got one. Whenever I have the vehicle still running, I would have to set the hand brakes then hold down a button on the remote to activate the remote start function. When I shut off the vehicle it is in neutral (level ground). Only then can I remote start it.
If I don't go thru that process, there's no way anybody, say a mechanic, can out of no where start the car up if I don't preset it.
What I fear more is when I take my vehicle for repairs and the technicians leave in in 1st gear with the car off. If one time they start the car with it in gear the car will lurch forward and may hit somethhing. I make it a point to notify them and have it written on paper to put it in neutral when off to prevent this from happening. Better safe than sorry.