Have we gone too far? Can you see any problems with this?
To be honest, yes.
Unless there was obvious evidence that a plotholder was failing to cultivate their plot altogether within a 12 month period, or not up to its full potential after 3 years, then by all means reconsider their tenancy and perhaps suggest a smaller plot. But
how someone cultivates their plot should be up to them - not some rule book of when this needs to be done by and how!
My plots are always in some state of flux - as I concentrate on one (like the half plot now as I get ready for the potatoes, onions & brasscias), the other (for summer fruiting veg) gets a bit neglected until I'm ready. Come June, the half plot will be ticking over with the odd look in while I'm planting and harvesting summer stuff, then come September, I'll be back at the half plot enjoying another harvest. This works for me - I have plots to grow my own food and that's precisely what I'm doing while I work full time and run a house. The plots get cultivated, but probably not the way people would rather I did it. I'd be very upset if I got a rule book with "warning" this "warning" that - it is so unnecessary.
You need to set a minimum standard that everyone of all ages and abilities can attain to within reason, and have a more neutral tone to the rules. And why should it be all stick and no carrot - get a bit of fun in there. Every June/July most of the plots on my site get a bit tarted up for the annual competition and inspection of the plots for the show ... introduce some sort of reward scheme to encourage people to try to do better.
And you want to be careful of waving the rule book about illness - some are not always obvious. You might just cross the line of discrimination. "Special circumstances" can be misread in so many ways and that alone reads "it's okay for them but not for you" ...!