Opening a port on consumer routers does not mean that all devices are open. Normally you forward a port to a host+port in the local network. In most cases some server which you control. All other devices are not affected by opening a port.

Should partly be possible to do. Matrix: https://github.com/spantaleev/matrix-docker-ansible-deploy/blob/master/docs/configuring-well-known.md

Lemmy is not working. But might be at some point: https://github.com/LemmyNet/lemmy/issues/2608 But that issue also mentions on how it works with mastodon.

You ever though about where the money from advertisers comes from? I would pay for Google if I would then pay less for products that waste money on “marketing” by paying millions to Google.