Changing the road network will be insignificant compared with changing the mind set of passengers who presently use EMA, LBA and MAN !
I think the passengers would fly from Donny, but if there's nothing suitable the state of the local roads is of no importance. Living on the east sideof the Pennines, I've used the airport a few times, in preference to the other airports.
LBA can be a nightmare to get to. From personal experience, the facilities are relatively limited. After our recent experiences there we won't be using that airport again. We suffered a delay of 30 minutes getting off the aircraft (apparently not enough steps and/or ground staff), further delays getting into then through the terminal. Then waiting a further hour and ten minutes to get on the so-called "every 5 minutes" shuttle buses to the car parks. No thanks, just not good enough.
MAN can also be difficult to get to from east of the Pennines. Both the Woodhead Pass and Snake Pass can take forever because you cannot overtake slow vehicles, other than in a couple of places. If you're in a line of traffic behind a slow HGV or caravanner, forget overtaking. If there's an accident ahead on either road, as there often is, both roads are treacherous, you may well miss your flight altogether. You can use the M62, but this adds a lot of extra road miles and can also at times be very congested.
If there's a flight from Doncaster, then I'd certainly prefer to use it because it is already easy to get to, with a couple of alternative routes. The car parking is very adjacent - walk out of the terminal and cross the road to your car a few metres away.
The flights have to exist first!