...the Isky literature has claims about how "oil restrictor" friendly the bushings are, ..... So I'm leaving the door open for another explanation.

I will give my context to that 'claim' ...these lifters are more or less 'engineered' for a big block GM but they are packaged by ISKY to be used also (which they can in some cases) for a BBM tie bar is the only difference...lifter feed restricting in a BBC is a common modification.

I feel the bushings in this motor failed because of a lack of adequate lubrication.

It is because in your explained application you are loosing the suspension provided via interrupted feed oil. the way you explain it the case is more oil pressure 'interrupts' lol dry running.

That's what I'm going with unless I hear a good argument to the contrary.
I'm not going to argue with that reasoning.

The cam will go back to Comp, the lifters back to Isky...... See what that they come up with.

This is far from any type of "extreme" application either....... .430 lobe lift, 1.5 rockers, 600lbs open pressure.
This may be why it lasted so long to begin with without proper oiling ...I know they will fail quickly. I will make a GENERAL sidebar comment based on what I have seen one of our local ALL-MOTOR racer engine builders do...they grooved the body of a !!!JESEL!!! lifter body to enhance oil supply...so try to reason a product with SPECIFIC and 'superior' engineering being modified to suit an application. More specific being redundant for clarity in giving my opinion i would both make sure the axle feed is covered at all times by any of a few means and that at least a continuous .043" pathway is achieved.