Order Admin does not populate fields from saved address

Hm, I can't reproduce this error on my local system. On my local system the address selector already calls uc_addresses_apply_address('delivery', this.value); on the "onchange" event.

Maybe flush caches will solve the problem?

Else, there could be a conflict with an other module. Does the error also occur on a fresh Ubercart install with only the modules enabled that Ubercart Addresses need to function? Which Ubercart version do you have installed?

I'm on Ubercart 6.x-2.9 too. I saw my test environment is still on Drupal 6.22, but I doubt this error is caused by minor changes in Drupal. I will check this out later.

So, you now have actually two issues. The first one is that the address selector tries to call apply_address() instead of uc_addresses_apply_address(). The second issue is that uc_addresses.js doesn't get included.

1. apply_address() instead of uc_addresses_apply_address()

This can be caused by an other module that alters the same form as Ubercart Addresses. apply_address() is the original function supplied by Ubercart, but Ubercart Addresses uses uc_addresses_apply_address() in order to support extra address fields. (E.g. for Extra Fields Pane).
Ubercart Addresses alters the form uc_order_address_book_form to put in uc_addresses_apply_address(). See uc_addresses_form_uc_order_address_book_form_alter() in uc_addresses.module (± line 1009) and uc_addresses_select_address() (± line 1121). It's possible an other module undoes this change by altering the same form. You can search for the string "uc_order_address_book_form" in the modules you use and see if one of them is in a form alter function.
An other possibility is that the customer had no addresses in his/her address book, although the functionality should also work with addresses coming from previous orders, but I haven't tested this much yet.

2. No uc_addresses.js

I've no idea yet how this is caused and why you don't encounter the same issue in your other installation. But it's clear to me some Ubercart Addresses code isn't called, as the module registers uc_addresses.js to load in the function uc_addresses_order_pane_address() in uc_addresses.ubercart.inc (on ± line 238).
As you also stated the edit form uses single delivery/billing id's (yeah, it should be double ones with Ubercart Addresses turned on), it looks like that somehow the original Ubercart order panes are used instead of Ubercart Addresses ones. Ubercart Addresses alters callback functions of the "ship_to" and "bill_to" order panes in uc_addresses_order_pane_alter() (± line 610 in uc_addresses.module). The callbacks are defined in uc_addresses.ubercart.inc (from ± line 204).
Before Ubercart 6.x-2.6, order panes couldn't be altered the way Ubercart Addresses does that, but since you are on Ubercart 6.x-2.9 this should work.

I'm curious how the two noted problems can happen. Hopefully, you want to dig a little deeper in the code to find the cause of the problem. Feel free to ask if you have any questions about specific Ubercart Addresses code.

Thanks for your compliments and I'm glad you were able to resolve your issue. :) Thank you also for providing clear information about your issue, without that it would have been harder for me to locate the problem.

I've updated my 6.x test environment to Drupal 6.26 now and I still can't reproduce your second issue (no uc_addresses.js included). Are you sure you had Ubercart 6.x-2.9 installed? I once had a situation where I thought I had updated a module (by moving the module's folder to sites/all/modules/contrib), but Drupal still loaded the older version. After some investigation why this happened, I discovered I previously had accidently moved the module's folder into the folder of an other module, causing Drupal to take that one and ignoring the second (newer) module instance.

Anyway, your original issue had been fixed now, so I'm marking this issue as fixed.