I believe your information is a bit wrongly interpreted, NFT. Miele has two reservoirs for non-plumbed model: fresh water and waste water. If I am not mistaken, combi steam oven from other makers do not even have waste reservoir. Miele has a drain outlet at the bottom of the oven connected to a pump. For non-plumbed model, waste water drawn through the outlet is pumped to the waste reservoir; for plumbed model, waste goes out the drain line. There will always be just a little bit of water left in the fresh water reservoir. There is no harm in leaving some amount there, IMO. I, however, do empty the fresh water reservoir every few days and wipe it dry. Here are some photos of my plumbed combi. The space for waste reservoir is blocked out on the left hand side with a plastic fascia. The water level is at after a rinse program, just tiny bit left. A pump would suck out as much as possible water from the reservoir during rinsing. For this simple design, the tube has about 1mm opening, and a pump cannot suck out water below that level. So, you don't have to worry about waste water and smell if you go for plumbed model, NFT.

I'd be hesitant to out it to the left hand side of the fridge in your drawing. In addition to getting the pans and trays of things you'll be cooking and steaming in and out of the oven cavity, you'll also have to open the fascia to add water to the fresh water and empty the condensate water container, assuming you're going with a non-plumbed in model. The clearances seem tight and awkward. At about 20 seconds into this video you'll see what is required for the water containers, note: even though it's an older model oven in the video the current model line up has the same feature. https://www.youtube.com/watch?v=8k5MzTNrfuY

Why would a plumbed version be any more of a problem when it breaks than a non-plumbed? Worst case if you replace a plumbed with a non-plumbed then just cap the supply & drain tubes. A non-plumbed has a pump to go bad and that is also the part most likely to fail. Plumbed uses house water pressure in place of the pump so is less likely to need repair to begin with. I can't understand $1500 for the plumbing except huge profit for the plumber & builder. When I added a coffee station in my studio it cost $700 and that was a 60' run for both supply & drain and included plumbing for both a sink and an espresso machine (Breville Touch). Are you sure they didn't say $150?

Page 183 of the operating instructions/installation manual for the 6705 states that the temp of the drainage water is 160F/70C. Checked b/c that’s the one we’re going to install and are prepping for installation. That’s the same temp my dishwasher drains at on the Hygiene cycle, which drains into PVC pipe. As does my washing machine, which has a temp range all the way up to 95C. Can’t imagine there would be an issue.