The fragment of a URL is not sent to the server, so that’s where such platforms usually store the key. That’s also the way cryptpad does it. You can thus share the URL and with it the key.

Of course, you still need to trust the platform. The sourcecode link at the bottom of the page links to https://github.com/timvisee/send who forked from mozilla/send and links back to the web page.

There are minecraft reverse proxies, so, yes, a http proxy will not work, but the general idea is still viable and doable with very little effort.

Set up a few domains all resolving to one IP. Run itzg/minecraft-router and use that to proxy the traffic to different servers based on the domain.

Also, they don’t even need a reverse proxy, but just resolve the domain name to the IP (in the simple case of one domain name per I0). That can be accomplished by hosting their own dns server, editing the hosts file or just pointing a public dns record at the private ip address, which will only work in their network,l.