A lot of questions!
Firstly, L2TP. "officially", AAISP have two kinds of L2TP:
* Failover - You can use your existing broadband credentials (abcd@a.N) to create an L2TP connection. When this is active, AAISP will give this priority and route traffic over it instead of the broadband connection. This is a standard feature on all broadband accounts.
* Standalone - this is the £10/month for 1TB service, and will be on a different login (and a different quota) to any existing services. So you can't use this to route your existing IPs.
In the past, I think AAISP used to give out free extra L2TP-only "lines" on your existing login which would come out of your existing quota, but I'm not sure if they will do that any more - but maybe something you could enquire about.
Secondly, USB - I can't help you with the issue of active cables, but I do think you should avoid USB modems, and just try a standalone router/modem. A few reasons:
* All USB modems seem to be quite old, meaning they have low "category" LTE modems. It's rare to find anything above Category 4, which excludes carrier aggregation (it was introduced with Category 6, and improved in the later categories).
* You will be limited in location to somewhere near your Firebrick. With a standalone router, you can place it anywhere you can get an ethernet cable to. You can even get external routers.
* They "just work"
I'm currently using a mini POE-powered router (AR-150) with USB modem (ZTE MF823) to give a failover 4G connection to my Firebrick. This is double-NAT (NAT in my mini router, NAT in the USB device), but I don't mind as it's just used for L2TP to AAISP. This is located in the loft, with the Firebrick in a 4G-unfriendly cupboard under the stairs and a 10m cable between them.
However, I'm looking to get a better modem supporting carrier aggregation, so will be replacing that setup with a standalone modem, probably the Huawei B535, which are selling at around £50 in ebay auctions, or around £100-£120 from Amazon.
But it sounds like for your setup (long distance from mast, thick stone walls), you may benefit from an externally-mounted router (or at least antenna), so I would definitely think about getting one installed professionally. Perhaps a satellite TV installer would do it for you.