I created my account here with a google account. Later I changed the E-Mail address in my google account but my beyond account doesn't recognize this change and still uses the old address. If I try to change my linked google account and select the same one again it just says that I'm already linked to that account, so I can't refresh it that way either.

It looks like the account linking system here is missing a way to update its data from an updated google account

I am having this same issue. I changed the gmail account that is linked, but all marketing and billing communications still go to the original address. 

Any update on this?
I recently left my job and I was using my Google account for my work (a mistake in retrospect). So, I no longer have access to the account associated with my characters.

How can I fix this?

With regards to Billing and Marketing your best bet is to contact them directly on the Help Portal or via the support@ email address, there is no other way I can see about changing such.