Yes, as far as I know, Amazon does NOT need you to have products live. It is ONLY the United States Patent and Trademark Office that will ask for “proof” of any kind.
Now, if Amazon DOES ask for proof, you can just wait until your goods are finished being made to start the trademark process through IP Accelerators.
But there is no reason not to at least try to start it now, because it means you will get everything sooner. Then you will know for sure if they require the proof.
I will actually be going through this process in the next few weeks as well for my new brand.
As far as changing your account to “seller account”….
Actually, don’t do that.
You will want to create a SEPARATE account to have a Seller Account.
Go to sellercentral.amazon.com and use a DIFFERENT email (ideally a business “support” email address) to create the Seller Account. You can see the steps to creating the account in our New Seller Orientation training (which I am pretty sure you watched already).
I don’t recommend linking your buyer account to your seller account though. There are a few reasons, but let’s just say it is going to annoy you later if you do this.