The SR520 is WMM certified, but it does not support diversity. For mixed wireless voice and wireless data deployments it should be a good choice. However, you are sayig that the PC's will be wired to the network, while the voice will be wireless. In this case, I think that more important than WLAN QoS is WAN QoS. The reason is that with this being a teleworker deployment, the 7921 will be registering to a remote UC500.
With regards to the lack of QoS/WMM support in the UC500, that has changed somehow. We still mandate that you deploy a WAP/WLC when using the 7921. But with the new SPA525 phone (wireless desk phone), CCA will push QoS configuration for the embedded and external access points, depending on the deployment model. Notice that this is possible because there are no roaming or mobile considerations (the SPA525 is a "static" phone).
Hope this helps,
Marcos