Most of my experience with pre-inlet locks is with Pecatonica Rivers stocks.
The mortice was done as I expected it to be. That is, it was undersize for the lockplate.
This is as it should be.
There are variations in these lockplates and by leaving the mortice slightly undersize, the supplier is giving the builder the opportunity to enlarge the cavity for an exact fit with a pointed razor edged Exacto Knife.
Had they made it so the largest lockplate would fit, it would be sloppy for all of the other lockplates.
Sometimes I get the feeling that people who buy these kits expect everything to be done so parts almost fall together. They don't, by design.
As for the location of the pre-inlet mortice, they should be in such a position that moving the barrel aft is necessary to obtain the proper location of the barrels breech plug with the lock.
Again, this is by design because different barrels use different breech plugs so they require the barrel to positioned in a different location relative to the lock depending on who made the barrel and which breech plug is installed.
Sure this takes some measuring and calculating and some work with a good wood chisel but no one ever said these "kits" were supposed to be easy.
IMO these "kits" are a starting place. Everything will need to be measured and adjusted to get everything to come together like it should.
If people realized this before they bought their "kit" and started trying to slap things together without doing the fine fitting needed, they would be a lot happier.