*** EDIT*** adding a mechanic like a PMB/PMD score to PCs (ala CMB/CMD stuff) would need a new print run in the core books or Pathfinder 2nd edition (hope not). *** EDIT ***.
what if the psionics were treated as a "player optional class" then if you play psionics the characters need to to make a quick and simple, one time calcuation of what you mentioned, the PMB/PMD.
Per haps as follows: cha bonus+ Int bonus + psi attack level = PAB ( psionic attack bonus)
cha bonus+ Int bonus + psi def adjsutment + wisdom bonus +10 = PAD (psionic Attack Defense)
if you take the attacks and defense types of 2nd ed where a defense has a better effect against a paticular attack (variable of -2 to +2) as the psi def adj, then that would give the flavor of the older style (whick I like) and the flaver of the cmb/cmd of pathfinder (which i like also).
psionic combat would be based on damage to power points as i ssaw mentioned above. perhaps dimiler to the sneak attack damage of the rogue?
skills/powers would need to to be activated and then used based on power point cost. I like the activation of 2nd ed where you had the equivilant of a dc to activate, but with the caveat of a 20 or 1 producing a critical success effect or a critical failure effect.
I have a halfling that had a bad time of the shape change effect, where he got stuck in the shape of an owl for a while. that was embaressing. Later he ran afoul (a fowl?) of a thieves guild and used the crit failure to his advantage to change permantly into another halfling. There were a community of halfling within the city anyway, so what's another halfling?