-- It would be great if I could find this check mark in the registry and remove it there

All the Send/Receive values are stored in the SRS file in a non-editable format.

--- Where can I downsize my Pictures?

I resize my images on my PC before uploading.

--- Then I configured Outlook with two email addresses, each with its own .pst file.
--- The Send/Receive Buttons ON/Cancel are back!

How exactly did you get the OK/Cancel buttons back? Why can't you duplicate?

--- uninstalled MS Office ... cleaned the registry and deleted all Office folders.
--- But Outlook started with all the parameters as before.

We are missing something. There is no way, that I can even imagine, where Outlook could "remember" any previous settings if all the registry and data files have been removed.

I know it's drastic, but maybe it's time to consider a Windows Reset?