What a mess - it really did a number on that board section. I agree that acetone would be a pretty good solvent for some of the char, but unfortunately, it may not clean it all off. Some is likely carbon particles (from burning up the coating) and Ta oxides (from the cap guts) splattered onto (and into) the surfaces, and no conventional solvent will dissolve it. The good news is that even if you can't get it spic and span, any remaining residue probably won't hurt the circuit operation after cleaning and repair.
Besides some solvent action, good mechanical scrubbing should help. I'm not sure if your comment about IPA is a question (will it work?) or a statement (tried and it doesn't work). I'd start with IPA, if you haven't already. Paint thinner or acetone are logical next steps. Acetone may be a little too intense to leave on for very long.
Presuming you don't want to remove the board from the RF assembly, which appears to have its feed-throughs soldered to the board, the trick is to be able to work on board cleanup without getting anything bad into the RF. IPA and acetone would probably be best then, since they're miscible with water and will evaporate quickly, while paint thinner would be a bit messy and slow to depart.
If the board can be readily removed, you have more options such as aqueous washing with detergents, and some gentle abrasive action such as an opaque toothpaste worked with - of course - an old toothbrush, to scrape off the particles that aren't too tightly embedded in the surfaces. Regardless of the solvents and methods, I always use old toothbrushes for scrubbing. Higher intensity solvents may dissolve or degrade the bristles and heads, so you have to work fast in some cases.
Another option, if you have access to the appropriate equipment, is to skip solvents altogether and do purely mechanical cleaning, like with a soda or walnut shell blaster.
Good luck.
Ed
