CR,
I don't hold with the given reasons for APD, but if there's to be an "environmental" excuse it would seem reasonable to raise the seat-capacity threshold a little, say to 50 or 70 seats. This would effectively exempt the supposedly cleaner, less damaging turbo-props.
I think those claims are based on the outputs of the whole plane - that certainly seems to be Flybe's noise argument. I don't think it really holds when you look at output per passenger - every airline from BE (Q400s + E195s) to U2 (A319s) to FR (B738s) claims to be greenest.
I suspect they all have certain claims to make - based on routes, loads etc.
Sticking to the domestic argument - FR have all but retracted from this market, I think MOL put the blame more on fuel costs than APD.
BE's Q400s may well have lower overal outputs if they create less radiative forcing - still a lot of debate on that one, don't think airlines want to accept it.
and would/might encourage more use of regional airports, either delivering passengers into a major hub for their holiday flights or with low-frequency direct flights to "business" destinations.
Any kind of hub-spoke is inefficient - especially a short jump when the journey can be covered by land (eg GLA feeding into MAN). It is accepted on longer haul and some European markets where the hub gives access to many more destinations, but the holiday market is always going to be primarily about point-to-point services to the most popular spots.
Biz pax want frequency - why it is hard to establish routes from BHX when it is close to LHR. Leisure pax can fill up seats to make some routes viable - essentially key business cities like MAD or BER, which also have diverse cultural attractions - but neither at @ BHX! To be honest, I've been very disappointed with the lack of interesting new routes from BE, considering the flexibility their smaller fleet should give them.
individual airports (and local authorities) would have an incentive to invest in a quality "passenger experience", including the interface with other modes of transport.
Unfortunately, that needs volume to pay for it - just look at the funding issues with the GLA rail link - even if the cost for a tiny section of track was extortionate!