1) APD in the UK isn't helpful.
While I agree that this is true as a general comment, in terms of a comparison with transferring at DUB, the APD saving only works if you have two separate bookings as I understand it. I assume this means collecting bags, checking in again, and then security and US pre-clearance. How long do you allow for that transfer when making the bookings?
Where DUB using EI probably is attractive, is for US destinations served direct from there but not from MAN, especially if through tickets are cheaper than flying from MAN with a US carrier to one of their hubs. Currently, I think this includes LAX, BOS and SFO. But as Shed points out, with TCX opening the first two of those cities from MAN next year, even that could change, at least for leisure travellers.