On point 1, you'll only base your plans on a 4 car voyager turning up on the day just on case.At least 50% capacity should be possible even with "social distancing" measures. And, since XC still haven't deigned to bother operating anything remotely resembling a normal service, most trains are either 5 car or doubled up. So capacity is a lot higher than 51 seats.
I don't think anyone is demanding that they sell Advance tickets. In fact in this situation that is the opposite of what's happening: they're claiming that flexible walk-up tickets are "sold out" (which is a contradiction in terms) and yet in some cases Advance tickets are still available.
On point 2, in my opinion as a ticket monkey "normal" passengers will most likely go for advances or get the coach.