The SOHO is very “good stuff.” But a better solution may be one of the MAX series, all of which have an integrated modem. A “cleaner” solution.
Good antennas already installed are a huge “plus.” I am not sure how many CAT12 USB modems there are out there. Peplink’s list of presently supported modems is here – https://www.peplink.com/technology/4g3g-modem-support/. However, Peplink has been good about adding others and the page I referenced talks about how to do that. I have to say, however, the world is moving away from this approach. Do you really want to attach two transmission lines to tiny jacks on a USB modem-- even if you can find one? If you do it it’ll probably work – but it would be a bit clumsy and rather fragile.
No and no. As a Peplink Partner I’d resist that approach if a customer wanted to go that route.
Absolutely and it works well. See here – Can I Use Ethernet Adapters on the USB WAN?
If I was in a position where I had a SOHO and I did not want to let it fall into disuse my solution would be to use a Max BR1 device in IP pass-through mode connected to the SOHO’s USB port via an ethernet adapter. (We sometimes do that with Balance routers in fixed environments.) However, the much “cleaner” solution is to just use a single Max router. I may also note that the Max routers have other capabilities the SOHO lacks, principally using more than one WAN simultaneously (which opens up a “whole new world.” 
) Maybe take a look here → https://www.peplink.com/products/max-br1-series/ . There was once a very clean and simple table that comprehensively compared the various models. I’ll try and find it and if I succeed I’ll post it here for you.
Edit: Here’s the comparison table – Peplink.com - Model Comparison. The table does not indicate the class of the cel modem. I suspect that in this table “LTE” means 4 and “LTEA” means 6 but I’ll be pleased to be corrected if wrong.