Schools are certainly becoming more flexible about boarding. When we first started, the school was 75 mins away (closest we've ever been), making weekly boarding possible but it simply wasn't available. The kids came home some weekends but not others, especially when they were younger. All at parents' expense of course, and no pecuniary advantage obtained. Now there's flexi-boarding where kids opt in and out on an ad hoc basis (2-4 nights a week) - must be a nightmare to administer. Remember also that in most secondary boarding schools, the children are in school 6 days each week.
If you claim for CEA (Boarding), I don't believe there is any laid-down limit on the number of nights the child actually has to stay in the school; I guess the offence occurs when the monies claimed are not passed-on to the school. That would presumably require some sort of positive action to achieve the 'receipting' of the Invoices for submission to the CEA Clerk each Term.
On the couple of occasions the kids have come home sick for a week or so, there has been no requirement to inform the CEA Clerk that I am aware of, and no refund from the school for nights away.
Edited to add that, as a Claimant, the MOD is becoming increasingly difficult to claim allowances from; perhaps that's part of the strategy?