First up, apologies on my slow response...

There is no official Nginx Webmin module, so if you want to use Nginx as your reverse proxy, then you'll need to use the commandline. I recall that there was an old unmaintained third party Nginx module floating around which may or may not work if you want to try that? Although it's probably easier to just edit the Nginx config via commandline...

Also Apache can also be configured as a reverse proxy if you want to use Webmin. I assume that would allow you to configure it as a reverse proxy, although I can't confirm. Personally I find Apache easier to configure via commandline anyway, so not sure how helpful that will be?

It's also perhaps worth noting that you don't actually need to use a separate server to act as a reverse proxy (if you don't want/need to). Assuming that your other servers use a "normal" webserver, such as Nginx or Apache, then one of them could be configured to forward the relevant traffic to the other server (as well as host the content that it already does) as required.

Using Nginx as a standalone reverse proxy may be a great option though if you intend to add additional services in the future and/or want to occasionally reboot the backend services without it affecting other services (i.e. leverage the advantages of redundancy).

Regarding your Nginx question specifically, the Nginx reverse proxy docs cover it pretty well IMO.

If you have issues and/or have further questions or problems, please don't hesitate to post back.