> What if you don't have a German translation of your site, don't accept Germany-specific payment methods and don't advertise in Germany-specific channels, but still ship there?

Shipping there demonstrates willingness, that is the sales contract (shipping to the customer address), advertising or localisation don't matter since those aren't a "contract" but the final sale with shipment is. It's quite clear-cut.

> What if you do everything in your power to stop German users from using your site, but the site is free and VPN detection isn't 100% reliable, so some users do slip through?

If you can demonstrate that you had active safeguards to not allow someone in Germany to use your product, and these users had to find workarounds to access it, you are protected.

There are good answers: if you willingly delivery a service or a product to a resident of a country you are subject to its laws.