Darren - I am sorry you had a frustrating trip. I have done the PHL-DUB many times and had mixed results myself. The good part is that you got to experience Ireland - what a great place to visit!
As for your concerns, I think you should write a letter. While many around here, myself included, feel that it may fall on deaf (or uncaring) ears, the only way to get any traction is to inundate them with our comments and concerns. The poor customer service is the direct result of how US treats its employees and I have been clear with them that I will not support an airline who does not support its staff.
The quality of the product is a bit more ambiguous and subjective since some carriers like Arelingus are said to be much worse. Nevertheless, one can often find a US-based carrier that offers a much better product at the same price as US and that does make me wonder. If they want to offer an inferior product, let the free market take care of it, just let them know that it does not suite your expectations and needs.
Finally, the FAs taking Envoy seats bothers me. I have thought about this before, and I do think there is something to be said for the hard work they do and the amount of time they spend on their feet. So I am not upset at them for taking the seats, but at US for not upgrading displaced or elite customers.
I've booked $600 trips to Europe plenty of times and watched 1/2 of Envoy go empty as I've boarded in the back. What does it cost them to bump up a CP or the family who got bumbed the day before? Surely not as much as losing their business.
Good luck, I hope you do get a positive response...but frankly, I dont expect it.