How critical is valve to spout measurement?

I'm going to my mother's house the end of the month, and thought I might change out her old Moen tub/shower valve for something new that meets current anti-scald technology because there will be a bunch of people in the house. Not a big deal most of the time since she lives alone, but could be with a bunch of people around; and, the one in there is ancient.

The current setup has the valve trim almost literally sitting on the top edge of the tub spout. This means that the drop from the valve is probably only about 3-4". was looking at the Delta installation instructions, and it says 8-18" for this dimmension. There is a showerhead. So, is it likely to give problems, or am I looking at more work trying to move the valve and fixing the tile? I'll look at some other brands, but from what I hear, this is a common spec. Any recommendations?

The only real limitation on the distance between the valve and the spout, regardless of what some manufacturer's engineer says, it to make sure the two do not hit each other when you put the trim ring on the valve.

The critical measurement is the trim plate. Each valve's trim plate is different. If you measure from the center of the valve to the center of the tub spout, you will figure out how much space you have to work with. Then you need to measure the radius of the tub spout and subtract it from the measurement you took, and that will tell you what radius of trim plate you can get away with. Best thing you can do then is go to a showroom and measure trim plates to see if you get lucky.

I heard running pex to the faucet could divert it to the shower head

(Nearly) a totally different thing, but yes, restricting the flow with a small pipe can do that. I was just wondering whether than minimum distance was something other than making the trim fit. My guess is that distance is required to ensure the trim can fit, but wanted to verify it.