We have been deploying Peplink equipment in Yosemite National Park for a few years, for residential use. PM me and we can have a chat (or a call) to review the particulars and the extent to which our (happy) experience with the equipment addresses your needs.
With some specifics:
The local cellular provider in the Park is Verizon - AT&T and the others do not have much of a presence in the Park (unless you happen to be in a fortunate and exceptional location). Verizon (and the other providers) have not indicated to us that the use of a multi-WAN router is in violation of their terms of service. Peplink cellular routers are certified on the Verizon network, so all should be well in that regard.
Yes.
And similarly, if you have multiple lines with a single provider (e.g., Verizon) then you can allocate traffic across these multiple lines so as not to max out on a particular line.
Be mindful of the experience that with a single provider, multiple lines in parallel do not necessarily give you the doubling of bandwidth you might expect. In the Park (and with Verizon) we have seen 0%-70% bandwidth improvement when adding a line to a multi-radio Peplink router. The bandwidth (and the improvement) seems to be highly correlated with the number of people in the Park and the time of day. I.e., general traffic congestion will at times dominate what bandwidth is available to you an adding another line/radio may be of limited utility.
On a good day we have achieved approx. 20Mbps up&down.
There was a thread on this topic some time ago, summarized at Live Event Streaming Setup - #13 by zegor_mjol
The additional question - will it work? Cfr. the above notes on limited vendor availability. We have not used our systems for VOIP, so I would be of limited use on that topic 
Cheers,
Z