WORKING!! (but with a few warts...)
I now have an FM GRC receiver working with my modified gr-osmosdr code. There are a few warts, but it is receiving and demodulating signals without errors.
For some reason the DC offset removal is not working as well in Gnuradio companion as it does in GQRX. However, unlike GQRX I can successfully demodulate FM audio and the FFT displays (pre and post filter) show the correct frequencies.
Another thing that's odd is the baseband gain and IF gain settings have no effect. The RF gain does, and I find it acts oddly, but not all the time. Sometimes it starts up with bad IQ balance and offset, but running up the RF gain then backing it down eliminates the image and reduces the relative DC offset.
I'm not getting any buffer overflows or underflows with my program. I think the overflows I were seeing in GQRX were due to mismatched sampling rates, which results in the bad frequency display and inability to demodulate any signals. I had overflow and crashing problems with the FM until I got the decimation ratios right and added the rational resampler before the WBFM demod. FM performance, by the way, is very good. I have an FM station a few miles from me and it causes other SDRs I have some overload problems that I'm not seeing on the SDRplay, making it easier to pull out weak FM signals.
I hope the tech team finds GRC file useful for debugging the gr-osmosdr driver and interface with GQRX.
The GRC code is available at:http://transmitter.com/sdrplay/sdrplay-test-FM-receiver-20150630-working.grc