https://i.delta.chat/ is not contacted when invite links are used inside of Delta Chat. The reason we switched from extended OPENPGP4FPR: to https://i.delta.chat/ is that such URLs provide a fallback for the case when the user does not have Delta Chat installed. Invite links can be opened in the browser and then the user will see the page with a link to Delta Chat download page.
The reason for URL format is not Android restrictions. That was the answer to your question for why the domain name in the URL cannot be the domain of the chatmail server. Any Android app can open OPENPGP4FPR: and Delta Chat even still opens OPENPGP4FPR: links. If you go to i.delta.chat link you will actually see OPENPGP4FPR: in the second step which has the same data as the invite link but using OPENPGP4FPR:. The reason we use https://i.delta.chat/ is to have a fallback for the case when Delta Chat is not installed.
Contacting WKD means you have to do an HTTPS request directly to the server of the recipient, which can log your IP address and the time when you requested the key. WKD requires that public keys are stored on the server and are available to the server. With Autocrypt and header protection the server will not see any public keys. Practical result of this is that if you migrate from one server to another while keeping your key the server will not be able to tell that you are still the same user.