u/GeassedbyLelouch

The context:
My father's partner is an absolute idiot, he has no idea what he's doing, but he believes to be the biggest genius on this planet and that he knows better than anyone else, refusing to listen to advice or whatever.
This is combined with his low literacy. He is barely able to read and writing is a near impossible task. This combination is a synergy of pure disaster.
On the computer he just clicks on random buttons, somehow expecting that's how he gets what he needs. For example, his bookmark bar has the same site bookmarked 5-8 times, I regularly have to purge all those bookmark clones. He claims it wasn't him creating all those bookmarks, that it happens automatically and that windows keeps changing everything on his pc. (he always blames everyone else, never is ever anything his doing).
I can list endless examples of this.
That's the kind of person we're dealing with here.

The password fiasco:
Today, I nearly had a heart attack because I got locked out of my gmail account. I thought I had been hacked.
I used the "forgot my password" option to change my password and recover my account, fearing that I'd find my account stripped bare, having lost everything precious to me in the last 25 years.
Fortunately, it was nothing like that. Everything was still there and everything was still normal.
I did get some emails from a shop regarding account creation confirmation. This was the shop my father's partner had been trying to make an account for last week, until I finally did it for him. There were also emails from google regarding changing my password and these had the same time stamps as the shop emails.
I immediately understood what had happened.
My father's partner had been trying to create an account for my father for that shop and somehow ended up on the log in page of my google account (don't ask me how) and had clicked "forgot password".
I called my father to verify my theory, and, indeed, they had been trying to make a shop account and "somehow" my email address popped up.
It wasn't maliciousness or a hacker, it was just pure idiocy of a moron who doesn't know what he's doing.

So, for the future, how to I prevent this from happening again?
Just telling them not to won't help a thing. They deny having changed my email and accidents caused by utter incompetence and ignorance can't be prevent by "don't do that again".
What even is the point of a password if anyone can just click "forgot my password" and change it??
I don't own a phone because I do everything through email. So any solutions which require a phone are not possible.

As a side question, I tried to reinstate my normal password since it wasn't hacked, but gmail refused because it was a recent old password. How long do I need to wait to be able to change it back?

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edit: the likely culprit has been found.
20ish years ago I was the only one with access to my father's emails to pay his bills. His email address was my gmail's safety back up address. But time passes and things change. My father's partner now "manages" his emails too, and through it he confirmed my identity when google sent a security alert about resetting my password.
I'll delete my father's address as the back up for my account and create a brand new address just for this purpose.