When selling on eBay, is PayPal fee before or after eBay charges?
PayPal's fees are before eBay charges.
Does PayPal email & eBay email need to be the same?
No, it can be different. Refer to below link for more info.
When I click on 'Link My PayPal account?', I was redirected to a PayPal signup page. How to fix this?
First, see whether you've added PayPal as your payment method for Seller Fees.
If it isn't, add PayPal as your preferred payment method for eBay's Seller Fees and you'll have no problem linking PayPal.
If PayPal is your preferred seller fees' payment method, you'll have to get in touch with eBay support.