Sending Google Calendar invitations from one's own non-gmail address (workaround)

There is no answer to the question
Google is pathetic and you cannot change the primary e-mail of an account that has a gmail e-mail account as its username. No, really, they are really that pathetic.

We must never add GMail to this account:

Adding Gmail

If you add Gmail to your Google Account, your account's primary username will permanently change to yourusername@gmail.com.

On choosing the required birthdate for this account (name not required, go figure)— If we used the company founding date, we probably wouldn't be old enough for contractual agreements needed to use the services. Averaging our ages together seemed too complicated, so i went for the International Women's Day that was a key initiator of the Russian Revolution.