75-250 is indeed a pretty large range. But not too unusual to do that. According to https://www.ysi.com/water-hardness that would be somewhere between soft and slightly hard (no particular endorsement; this was just the first site that I found online). I don't think you'd go horribly wrong by telling your oven to treat this as "medium". Worst case, it prompts you to descale the unit more frequently than strictly necessary. That'll waste a little money on supplies. But if in doubt, that's better than wasting money on repairs. I recommend you buy the descaling tabs in bulk. They regularly show up on Amazon and other online services. And they definitely get cheaper if you buy a bigger package. You'll need two per descale cycle. I usually need to do so every six to nine months. But that might not be the same for you. We use the steam oven very frequently, but we have very soft water. Also, you absolutely want to make sure you have tabs on hand, when the unit needs it. You don't want to find yourself in a situation where you can't use the oven for a few days while waiting for supplies.

Yes, 6805 has a drain line. Miele non plumb (6800 for example) has two containers, waste/condensate container and fresh container. For 6805, there is only fresh water reservoir. Plumbing takes care of filling the reservoir automatically, as well as draining out all the waste. In descaling program, descaling tablets must be dissolved in the reservoir manually. Half way through the program, small amount of left over solution must be emptied out manually before the machine refill the reservoir with fresh water and rinse itself. Emptying the reservoir after each use is mainly for limescale prevention per manual. I have soft water so don't follow Miele's instruction of emptying to after every use.

I don't know the answer but I am curious. if I were you (and having the benefit of hindsight) I wouldn't hesitate to call any of the Miele USA "Experience Centers" and ask to talk to any of the design staff. If you are in the US start with the Princeton, NJ center, their US headquarters. Even better if you can get to a center. The staff at these centers are a world apart from anyone you'll encounter at any appliance dealer, they use the products for demos and classes and typically have the appliances in their own homes. And it's their job to know the details on what you are asking. And report back and let us know. P.S: A little searching and I found this: https://www.mieleusa.com/media/ex/us/PressReleases/2021/9.2021_Generation_7000_Launch.pdf. Food View sounds interesting. There's also this thread: https://www.houzz.com/discussions/6229141/miele-7000-series-speed-combi-steam#n=10 Advice on calling the experience centers still stands.

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?