The goal posts keep getting moved - to use the old hoary line -' the more things change, the more they stay the same'
Perhaps the club ( and maybe MA, soon, too?) has gone back to the old rules (not that I know Exactly what they were...) of a 250, of any sort, Not being allowed to ride in the 500 class? It came from the 60s when CZ/Husky turned up at 500 GPs on 250s and started to beat the up to 500cc 4ts - they then introduced rules that made it that it had to be something like 292 /300cc. Over the years I've seen 251cc, 256cc rules.
But , if a class is referred to as "open", it should be just that - open to all capacities.
Perhaps there's a few people in the club that are pissed off that they have 450s and can't get many class options themselves, so now that 250s are rightfully allowed against 250s, they've gone, stuff those blokes , we're not letting them get 2 classes out of one bike..........
Years ago, when I rode my hybrid CR/XRs, all I wanted to do at club MX races was ride (like you obviously want to do), and I was more than happy to pay for entry for each class I raced in. Some clubs couldn't handle that, until I got them to do the sums and see the extra money they would get out of me and others. Maths can be a bit of a problem for some...
I'm familiar with clubs wanting flag marshals - it's always been a big problem, and many will require a flag marshal from a rider, or a rider to flag some races - it's become a common practice. I can see it being a problem if you were wanting to do a lot of classes - as whatever rules about marshaling would still have to apply, to be fair to others.
I'm a bit puzzled by your point about being chastised for not having parental supervision at 34- either you must look very young for your age, or you might have been 'acting' a bit young at the time? Were you loosing it at them?

In all my travelling to and from the states in years gone by, I too found our set up here infuriating, but I found the seemingly, to me at the time, completely uncontrolled licencing requirements in the US pretty bizarre too - but I sure liked being able to turn up at a track in California and basically pay my money (and not much of it, either) and race / ride. I think, at first, I was so stunned by the ease of being able to race /ride, in a country 'famous' for it's litigation happy rep - after spending much of my life under the ACU/MA jackboots - I thought that I was in some sort of 'fantasy island' scenario.