I have never seen a built-in oven that has been installed for 30 years or more cause yellowing of anything but itself (if its white -- white plastic controls on any appliance usually yellow). Manufacturers wouldn't spec their ovens to be installed under other appliances or counters if they weren't designed to do so. If you were installing say, and Advantium 220, that could cause damage. It specifically says not to install it under another appliance or under counter.

The space that you lose by putting in a wall-oven, you gain back under the cooktop. Put three drawers under there and you're golden.