Some very thought provoking stuff.
Would I pay to ride a trail centre - yes and no. I'll happily pay to park at Llandegla but really objected to the £7 blanket cake hunting charge at dalby. It just wasnt good value for money. Paying for showers, bike washes - no problem.
I'm very much against permits to ride and access charges - its a step on the road to restriction. Given we are a society that likes to sue direct charges for access are riding could have implications regarding liability. Someone is going to try the well I bought this and it hasnt performed as expected.
Maintainance costs really shouldnt be an issue with bigger centres - if they have got their financial planning right. Cyb, Dalby, Llandegela, 7 stanes etc I would expect the repairs and maintainance to be funded from a proportion of the revenue generated by bike hire, bike wash, showers etc, bike shop franchise, cafe franchise, car park charges.
The solution to Penmachnos problem could be some form of national trail fund. Simillar to the one set up for Fix the Fells in the Lakes or the Access Fund in the States. Asking the village to pay for keeping the trail would probably take any additional money brought in straight out again. It may be a grosse simplication but I suspect most people who ride the trail dont spend any time let alone money in the village. Unfortunately if you build it they may come but you cant control where they spend and much of it is probably going into existing tourist businesses in Betws.
If a central fund was established it would need to be independant - a public or private land owner tie in would probably be seen as some form of subsidy on things the public think they should be delivering already. it doesnt mean work on those trails would be inelidgable. It just means it would be the volunteer groups that could apply. Yes there are already a lot of grants and funding sources out there but normally they dont fund ongoing maintainance.
So its taken me an hour to write that I'm still not happy with it but I'd say I'm in dont charge to ride do try and find a way of encouraging all who benifit - indviduals and businesses from local to multi national who benifit from the presence of trail centres to support.