P40 : TF does stop an element recoiling unless "(e) a PF or TF it has not already crossed"
I think this would apply to pushed back elements too.
If it has not previously crossed the fortification, then it would prevent the recoil.
The -1 combat factor is if recoil is prevented by "troops, terrain or a battlefield edge" and a TF is none of those. Therefore by the letter of the rules you don't get a -1 in the first case. When you have a column behind you, it is troops that are in contact with the rear edge of the element in combat, preventing the recoil, so the -1 does apply.
IMO it is intended that a fortification preventing a recoil also gives a -1 - I can't think of any reason why it shouldn't.
Lawrence Greaves