A few thoughts:

1. Good that you are helping your dad - but be sure you are actually HELPING and not just complicating things. You do this by keeping HIS needs front and center.

Him being able to do the laundry so he remains independent is far more important than the laundry being beautiful. Don't impose your requirements on the project that aren't really relevant to him.

2. If you can make the redesign work better for his needs AND look better than it otherwise would, that's great. If you can't do things that look good, then think in terms of it being temporary and able to be easily "undone" by the next home owner. A pre-fabricated cabinet in the dining room with a stacking washer and dryer may not look great, but can be easily removed, plumbing and electrical removed and wall patched, etc. whereas a framed-in closet is more difficult. Avoid cutting old trim, hardwood floors, etc. if it's going to be temporary.

3. Work WITH the contractor rather than imposing demands long-distance. Your dad is the one who is going to bear the brunt of a pissed off contractor, not you. Use lots of diplomacy.

4. What is the reason for wanting access to the shutoff valves? If it's for leak protection, consider something like this valve that shuts off the water supply when the washer isn't on. It also has a leak sensor.

Amen to Wonderdog.