Those CR-915 radios can be a problem. I've been through five of them in my '95 940s before giving up and going for decent aftermarkets with proper modern features. A recent discussion here concluded once again they can't be repaired. A frozen or dead panel or a failed tuner section are known issues I've had.

Now having said that as a caution to you going forward, if you're stuck at CODE entry and not getting a button response then that's normal if you're not putting in the correct code. Because the radio has been changed the original code in the owner Radio manual is no longer correct. The new code should have been written in either the Radio manual, Owner manual or even Maintenance manual, so double check those. Another place to check for the code is handwritten on the radio, but dealer service is not supposed to do that. The sure solution is to take the serial number on top of the radio to a Volvo dealer who can contact Volvo North America for the code. There should be no charge for this service. They can also lookup the code by VIN so you don't have to remove the radio, but of course you know it's not the original. You've already figured out how to deal with the OFF lockout.

Good luck with your new 940, looks like a peach. Change all the fluids (oil/coolant/trans), check the brakes, remove and clean the throttle body and PCV system, inspect the plugs and you should be good to go. I wouldn't sweat the T-belt. You can inspect it for signs of aging if you want to, along with all the other drive belts, but I wouldn't get too worried for such a well maintained car. The later round toothed T-belts have a longer 100K service interval, but with the age factor here I'd change yours closer to 50K. 10K is an inspection to make sure the belt remains snug and was properly installed. The intention was to be able to remove the rubber knockout, loosen the tensioner bolt and retighten it after the belt has had a break in period, but I'd rather have the cover off. I trust my handiwork and quality belts don't need a break in, so I never bother. I'll probably have the cover off for another reason soon anyway.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now