Pixel Phone return - Google store

No experience of Google trade-in / returns and don't quite appreciate the scenario but the observation would be there is no statutory "15 day return window" for specifically a faulty device - a faulty device has a statutory maximum 6 years action window that may or may not involve a return, unless you live in Scotland.
And any off-premises contract cancellation period is 14 days.

Was there not a replacement opportunity, or is that a silly option and part of my not fully appreciating the scenario?

But to quote a Google one-liner summary prior to its contractual t&c:

...If you cancel your new plan or return your new Pixel device, you will not be able to retrieve your old phone and will only be offered, by way of refund, an amount equal to the trade-in price given for your old device.