If you miss the 45 day deadline even by one second, neither Paypal nor eBay will go to bat for you.
It is a good idea, because of time zones, to open Not Received disputes no later than Day 43 after purchase.
Don't worry about the seller. The Dispute actually allows him an additional 20 days to resolve the problem.
It would be polite to contact the seller first, but do NOT miss that deadline because he is "investigating".
Either he has used a Delivery Confirmed service and can show that the item arrived safely or he cannot. All he has to do is put that number into the dispute.
PYA.
If you are refunded, take your time about feedback. The item may arrive, late but safe. In that case you can leave appropriate feedback (you have 60 days) and -- this is important-- repay the refund through Send Money on Paypal.
Neither of you can ethically have both the item and the money.