Obviously I couldn't comment on the plumbers first hand knowledge of your situation and plumbing issues he refers to. But you have a sink in the blue room so surely that will get blocked also if he is concerned about blockages.

I would love to move our wm and drier upstairs but our last house was a hundred yrs old and my husband felt the floorboards wouldn't cope with the vibrations of 1600 spin so hay stayed in the kitchen which had a concrete floor. Our current house is new and, again, he felt the chipboard wouldn't be solid enough and they're in the corner of the garage. In the US, a laundry cupboard on the upstairs landing is de rigeur!!

Yes you can. You will need to ensure that the dryer is sitting on a solid piece of top. You will also need to take into account plumbing behind the washer so you will need a deeper carcase than the standard 600mm deep. That will in turn involve deeper tall end panels. I have mine mounted in the same situation and find it much easier to get the washing out of the dryer. Here are 2 images of mine, one with doors closed and the other with doors open so you can see the appliances.

Miele machines do vary considerably in price. Is the bottom of their range within your budget? They really are great machines. l can only comment on other appliances by the two manufacturers you mention. l have a Neff double oven and wouldn’t buy another. It cooks more on the left as opposed to the right when baking cakes. l also have a Neff fridge that goes into a semi freezer mode when it takes its fancy. l got rid of my Bosch induction hob. Swapped to an AEG. Much better. My preferred brands are Miele and AEG and l would also consider Siemens