[mothered]

it's better to  have the PR already before I contact Amazon CS?  I'm thinking it may look like I'm being proactive about the situation thus increasing the chance of success and also speed of the refund.

It can actually work against you.

As far as you're concerned, your SE Is a legit claim and as such, you're simply a customer expecting a refund/replacement for an Item that didn't arrive. You're not supposed to know that a PR may/will be requested, so having It on hand before events take place with Amazon, can be seen as an Indirect admission of guilt.