If you can change your DDNS, yep. You can also use the LetsEncrypt staging endpoint. In fact I’d highly recommend that. Or turn auto restart off until you’ve got all the config nailed. Or all of the above, really.
Can’t be… Rewrite the proxy directive out again, something might be mistyped.
I just started up an instance of Caddy 0.10.3 on my PC with a proxy / some-imaginary-backend:80 (literally) and it runs fine. Needless to say there is no some-imaginary-backend available on my network. Conks out when I try to access localhost:2015, though (error 502).
Worst case scenario, remove the links entirely, change the upstream in all your proxies to your NAS IP on the LAN, and try again.