Hey @Gordon, in general, TTN shouldn't differ much (at all?) from any other LoRaWAN network. I haven't tested myself, but there are a bunch of guys in our local community that have sensors that they interchangeably switch from network to network without issues.
If you wanna setup a local test environment to isolate from TTN and any other provider, I'd recommend this backend: https://github.com/brocaar/loraserver It's well documented and I know others have tried it successfully. About the concentrator, I guess you have it figured it by now, but you could use TTN install scripts (https://github.com/ttn-zh/ic880a-gateway/wiki) and just change the backend server to forward to on the local_conf.json
The biggest problems you will face with downlink (from backend to espruino) are related to time sync, the receive window is rather small, and it's a source of constant issues on some MCUs.
Let me know if I can give you a hand.
EDIT: Actually, I DID test switching networks now that I remember, and it worked without having to make changes to my node. Just provision the keys on the new network and off you go.