Well, my saga is finally at an end. Having received my trade-in on 10/30 with a quoted turnaround time of 5 days, Google decided after 30 days to do the grand courtesy of emailing me on 11/30 apologizing and promising to complete it in 10-15 days. After that passed, and with no further notification or follow-through on two separate escalations they committed to, my device was finally assessed and credited over the weekend, for the full quoted amount (though I have to wonder what person at Google thought it would in any way be construed in a positive way by the customer to say "its value either matches or is more than the estimate," which is essentially letting me know that, best case, I got fair value, but there's certainly a chance I got screwed because in their estimation my phone is worth more than what they quoted (if they reserve the right to reduce the estimate, with that verbiage they should also maintain the possibility of increasing it).
Oh, Google's evidently official position on what to do if Google escalates and requests but is never provided an update from the 3rd party company? Wait 30 days and put in a new request. No next level escalations are available to them, and per them they neither can provide nor have themselves any telephone information for the 3rd party, nor can they provide email address or even identify the name of the company used.
Yay.
My advice to those considering the trade-in is that with processing times ranging from about 1 to 8+ weeks, consider the value of simply selling it outright if you can get reasonably close to value. If you're buying it with a credit card, your interest-free 'grace period' (typically 30 days) may well expire before you get your credit, meaning you'll either have to pay the full cost of the phone and then wait for the credit to come back at some point, or suffer interest charges.
The new models are still fantastic, but my recommendation to any who ask about the Pixel line has gone from a 9/10 to a 7/10 simply based on the need to lower expectations of quality of customer support, given how this process has gone. While my prior experience when needing assistance had always been stellar, if Google is slipping this badly in this aspect I have to anticipate that they may well slide in others as well.