[mothered]

from a third party seller selling from amazon

Is It a company or an Individual seller?

the sealed box method

Is the packaging fully covered In cardboard, thereby the GPU cannot be viewed externally?

Regarding the sealed box method;
If the item comes with warranty sticker-style peeling stickers as the only seal preventing opening of the box - would a successful removal of the stickers in their original form, followed by exchange with an item of same weight, and successful reapplication of the sticker (again in its original form) and a return, be met with a check of the item inside after the return process

On the grounds there's no signs of tampering, the objective of the sealed box method, Is to have your return placed back Into stock and a refund Issued thereafter.

In short, don't give them reason to check your return.

Also, if the item is checked and the SE uncovered, what may the consequences be?

Nothing.

As far as you're concerned, you've returned what they've originally sent. Another SE'er could've used the sealed box method, and you just happened to purchase his/her return.

When using the wrong item method, what implications does a highly priced item bring - would amazon be inclined to raise a police report?

The method doesn't warrant a police report.

It's not a theft-related Incident, hence a PR Is Immaterial.

I'm also curious about the EB method - is this still feasible with a high item weight (~1.6kg)?

If an Investigation Is opened, the claim will be Instantly declined.

As stated above, the maximum weight Is around 120 grams- and that's pushing It to Its absolute limit.

Finally - if the third party seller also offers PC repairs, is it logical to assume there is an increased risk of the SE being uncovered (in the event that the GPU is used for upgrading / repair)?

On the grounds you've SEd the GPU from the third party seller Itself, how will It reflect on the repairs offered by the same seller?