"1. Why is one simple LCALL causing my 8051 to hang/crash?"

I imagine that the address that is being LCALLed has not been calculated correctly because you are trying to use the eval version outwith its limitations. I am surprised, though, that the program compiles and links successfully if this is the case.

"2. What are your recommendations for dealing with this situation (in general) and functions called in C?"

You're not going to like this, but the solution is probably to buy the compiler. However, I think it would be worth contacting Keil and explaining your problem. If the eval version really is producing dud code without complaint they will probably be quite helpful.