[Kalun D]

"" noted a fuel leak at the base of the #3 injector (TWM set up). Pulled it, and lo and behold, no tip, just the o-ring and plastic 'separator?' hanging loose. ""

What brand of injectors?. You need to pull one of the other injectors, compare and determine what exactly fell off. Metal, plastic or both. A fuel leak indicates a "broken/bad" injector. I'd examine what may have caused this one to break, and maybe replacing them all with another brand.

""The valve area (exhaust) looked like it had black bubbly froth (hardened) around it. ""

normal soft carbon can look like black froth, you need to be running Chevron or any fuel with cleaner if you have build up.

""(compression test = 0 psi)""

that's not good, a leak down test will tell you which valve it is, more accurate than a comp test.

If a valves bent heads will have to come off. You could remove spring and spin valve while observing with borescope for runnout. There's also a possibility although not likely that plastic from the injector has melted onto the valve seat causing loss of seal.

The fact you heard noise isn't good and says "metal debris" to me.

""Does anyone in this august body have any suggestions? Any chemical engineers know of a solution that will eat plastic and not steel and aluminum?""

The plastic on most injectors is nylon. Don't know what melts that and not aluminum but there's probably something. The idea of a small propane torch might work also.