For what it's worth, I am the Chairman of my Parish Council, the webmaster of the parish website, which is a whole community site, not just a parish council one, and I'm also the editor of the parish newsletter. We don't provide any direct feedback options for residents via any of the above (letters page, blog etc), neither do we have any parish council social media outlets. Experience has shown us that offering a soap-box in this way encourgaes ill-informed attacks from keyboard warriors that benefit nobody. We encourage people to contact the chairman or clerk by phone or e-mail or to attend a parish council meeting in person to air their concerns.
It will, from time to time, be necessary to criticise your district or county council, but this need not be expressed in a negative way. As a parish councillor, part of your role is to lobby other authorities on behalf of your parishioners. All decisions of district and county councils are published on their respective websites, so they can be discussed openly. Almost nothing is off limits these days, and rightly so.
Criticising your parish council, as you suggest, is more complicated. As a parish councillor, you accept collective responsibility for the council's decisions and actions. If you're not happy with things, you can use your position to try to change them, but you should act in the best interests of the council at all times. I don't see this as being incompatible with the role of newsletter editor. Indeed, your council might benefit from your dual role in being better able to communicate with parishioners.
I'm also the clerk of a couple of other parishes and, in one case, the parish council has been forced to consider taking over the responsibility for the parish magazine, as an out-of-control editor abuses his position to cause trouble, attacking not only the parish council, but individual, identifiable residents and whole streets within the parish. This is so serious that it has now become a police matter.