Thanks defragster, I actually made a typo when I wrote 1.5.8B, I meant C. All of the previous talk about B must've gotten to my head.
Anyways, to report back, correctly this time, 1.5.8C does not run on my Macbook running OSX 10.6.8 - however, when booted to Ubuntu 14, the linux build works just fine. This isn't my regular setup, but I can use it as a temporary work around.
The issues I previously reported have continued into 1.5.8C when using OSX 10.6.8, and in fact, on the original beta release of software, I saw more functionality. The IDE is very slow and laggy on 10.6.8, which I noticed carried over (less slow, and less laggy, though) when I tried 10.9 - maybe it is not optimized for the systems I have been using but it is significantly slower than the stock Arduino IDE even when working with UNOs.
I have been able to get the IDE to work with boards other than the Digispark PRO, however, as I mentioned before, it is slow. I've tested with a generic and real UNO, an SMD generic UNO, several Pro Minis, and a standalone chip using an UNO as a programmer. I've been able to successfully upload in all of these cases, but I have noticed one other issue - almost every time, I must quit and reopen the IDE or it will not upload a second time to the same board, even when I have physically detached the board and reattached it. It worked fine with the Pro Minis, but I'm skeptical because they are generic boards.