We have been in Owners Suites on two Vision class ships and they were very nice. I peeked in the Grand Suites and the big loss is in the seating area.
Yes, only those in GS or higher get to order from the dining room menu for breakfast.
If you have a reservation for My Time Dining but want to change it, you can do so up until a certain point on the day in question. The phone number will be listed in the Cruise Compass, but if you do go with the suite just ask the concierge to change your dinner time. That is what he is there for. Or you could simply show up when ready to dine without a reservation, but you may have to wait 20 minutes or so at peak times.