Ok, I may have made progress on the too many file handles issue. I found and tested where the linux loader opens its file. The Mac loader should do the same. The file(s) are no longer kept open.
Currently committed in trunk r59729.
If you could please test, and confirm if the problem with "unable to open file" is resolved?
For testing, set your file limit lower (not sure what else the IDE needs (project, units, lfm, lps, codetools may scan units that are not open...), but maybe try 30 to 50). Also do some debugging with this setting, to see if there are any issues while you inspect data (watches, locals, hints, opening new editor files (blue dots), ...)
Thanks for testing.