Keep hl2.exe in the whitelist.
start mumble before you start steam
disable steam in-game overlay
try this snapshot version of mumble 1.2.4? http://mumble.info/snapshot/mumble-1.2.4-rc1-12-gdab8157.msi (if you haven't already)
combine all that with -nod3d9ex, -dxlevel 81 as well as fullscreen with your height and width defined (or borderless windowed if it doesn't work), and start mumble as administrator. If all that combined doesn't work I hope you're good at memorizing voices.