1) can I make mentioned miniport driver using WinPkFilter?
In case of using WinpkFilter you even don’t need to create an extra miniport driver. You can intercept RTP packets between the physical network card and TCP/IP stack, set the QOS bits, recalculate checksums and re-inject packets back into the stack.
2) Does application need admin rights to load this driver?
You need admin rights to install the driver, not to use.