If your serial mode is 1 or 3, you can simulate the serial port "at bit level" using UVI51.
Unfortunately, UVI51 is not able to simulate 15,000 lines of code, and you would have to test only a piece of code of about 4000 lines maximum.
I have simulated a serial link between 2 micros with UVI51: One of them using the embedded UART and the other one using a SW UART. UVI51 lets you see how the bits are going out and entering the shift registers.
UVI51 also includes a logic analyzer where you can inspect the serial frames, measure times, etc.

I can send you a free copy of UVI51.

Alfredo del Rio.