1. In this case, you must have logged in with the admin ID. and the admin doesn't have selling permission and he doesn't even have a store, so it can't take you to the shop of admin.
2. I didn't got such problem like that. In your sellers page, currently showing two active sellers. And I have checked through all the products and the sellers page and been through the links back and forth. And couldn't find any problem.
3. I have also checked this too and everything is working fine.
I have only found a issue with the seller named “Peter Ollouson” and that doesn't exist on your users table. Maybe you have changed the user role later. There's only 3 sellers including mine demo seller “John”. So, according to me, everything is working fine. Please check again carefully with a seller account, customer account and without logging in.
And please remove the previously created products. So you won't have a broken link.