It would work the same way it used to back in BR days, when you paid extra for a reservation! The quota control on "booked train only" seats per train would be the same as now, just there would be no guarantee you'd get a seat and you wouldn't be allocated a seat number.
I'm really confused now. You said a "guaranteed seat scheme". "Counted places" does not guarantee a seat. How would the "guaranteed seat scheme" work other than by (optionally) reserving one?
I do think Advances should have a reservation, though, other than the sort of "edge case" below. The reason is that unlike a walk-up ticket I have no choice to say "this train is too busy, I'm going to take a different one".
We need to get away from the sort of nonsense that prevents long distance XC passengers being unable to travel/book a cheaper ticket simply because half a Plymouth-Newcastle service has seats booked Bristol TM-Bristol Parkway.
There are a number of possible solutions to that that don't involve losing the benefit of seat selection and reservation.
1. German-style, don't allow (or charge a hefty fare for) local travel on IC services except where there is no local alternative. On DB, this is usually handled by running ICs as REs for part of their journey where there is no alternative.
2. Don't allow reservations for short journeys, but do have them for longer ones. I agree stuff like this (and Manchester Oxford Road-Manchester Piccadilly, which sometimes happens) is silly.
3. Provide XC enough capacity.