A follow-up. New problem. Although engine now quieter, vibration less, and no more 'knocking' while starting - all due to the new motor mounts - there was a rattling noise, intermittent, that I could not localize. It appeared to be in the fan belt path. At one point today after pulling the car back out of the garage, I discovered a small bolt in the garage under where the front of the engine had been. I recognised it as the stripped-head torx bolt I had tried to re-use attaching the damper to the idler support.
By now it only took a few minutes to dissassemble to the point of replacing that bolt with a NEW allen-head one, but tragedy finally struck - the tensioner broke while resetting it for the umteenth time. The broken one came out with three bolts attaching bracket to engine, and the 17mm one through the middle, after pulling the power steering pulley for access. Hopefully the new one now on order from Phil will go in as easily.
Tensioner failure mode was as others have noted, where the rubber hardened and cracked, preventing normal tension from being possible. In addition, adjusting further I cracked the torque-lever that turns it from the adjustment screw.