I don’t see any obvious issues with this deployment scenario. I would expect the LTE circuit will have less bandwidth availability than the FIOS Fiber, so customer expectations will need to be set correctly as to what experience they will have in a failover situation. In similar situations I tend to install the router utility on the office managers smartphone so they get push notifications when the primary WAN link fails. That way they can inform the office staff and manage the human element of bandwidth usage.
It would be worth setting Queue assignment based on DSCP and class of service (802.1p/CoS) on the Cisco SG300 to prioritize the VoiP traffic from the Polycoms over the network, and then create a custom rule in the QoS settings on the 305 to prioritize that traffic over the WAN links too for the best possible Voice experience.
I have never heard of any regulatory or legal reason not to run SIP over wireless. Most people have historically prefered a wired connection for enterprise voice for obvious reasons, but VoIP over wifi is used by millions every day (think Skype Lync and 3CX smartphone clients used over wifi and cellular) so I doubt the veracity of statement.
The 305 is a great bit of kit, and would allow for future use of the 3 WAN connection and comes with all the enterprise features like inbound load balancing, user group bandwidth control (so you could set a bandwidth reservation for the VoiP handsets if they were on their own VLAN) and support for up to 20 remote VPN Peers. The only alternative product you might consider is the Balance One (only two WANS + USB, and 2 Remote Peers with no inbound load balancing or user group bandwidth control). Take a look at the comparison here to see if it would work for this deployment or not http://www.peplink.com/products/balance/model-comparison.