I have tested and these are my findings:
Good:
In such tests I usually install packages in groups of 1-2 letters, like first all starting with A and B, then all starting with letters C and D, etc. This time I have reached P before incompatible PPU stroke again. I do not think I have ever reached this far without applying the workaround. Impression is that I have been able to install 2-3 times more components then usual. Of course, there are some components that do not work but I just skip them since they do suffer from compatibility problem, not from PPU problem. After I faced the PPU problem, my workaround worked and I could continue.
Bad:
I did not measure, but my subjective impression is that compilation is now much, much longer. Like IDE and all components are compiling again whenever I have new group of components for installation. That might not be that bad if you select everything and just select to skip all errors, but I installed in group of 1-2 letters and that increased time exponentially (subjective time evaluation). After half of the letters were installed, I had to let it install and do something else and only check from time to time if it finished or showed an error. It was thaaat long. Another bad thing is that some component near the end of the alphabet breaks the Lazarus completely and it can be started only if you rename lazarus.old.exe to lazarus.exe. I will see if I can identify problematic component, since that is not good at all. Imagine poor newbie who didn't make a backup and he can not start Lazarus any more...
Conclusion:
I do not have a definite opinion about your PPU fix put into OPM. I think I can compile much more then before without PPU error striking, but it seams to make compilation much longer. I think I would prefer OPM without it, but I am not 100% sure. I really think that someone from the core team should take a look at PPU problem, and I have already tried with bug tracker but it made no difference. Bug can be repeated but you have to spend several hours just to come into position to catch it, and it strikes. Like a hot potato no one likes to hold in his hands.
When I face this problem again, do you think that I should try to install packages again manually in hope to reveal if it has anything to do with OPM?
Yes please. It would be really helpful.
After PPU problem I tried to manually install already downloaded package which had reported incompatible PPU and error was the same so I could not install. After manual workaround everything compiled and I could continue.
Btw. I think it's a little odd to be notified before installation that packages are not compatible with user's Laz+Fpc combo. I would prefer that instead of this, OPM says something like packages have not been tested on that combination. It's a little less scary message.
Thanks for the suggestion. Done in r.61850. Please test. As a side note, the dialog can be disabled from option. It won't show up, even when you try to install an untested packages.
Nice. Thanks!