Spend enough money and you can do anything you want!
OK the present diesel service has been taken as far as it can go, are you saying it would be impossible to take it to the next level?
What I'am suggesting is not a piecemeal scheme but a Bathgate style re-vamp where a half-hourly 2 car diesel was replaced with 4 3 car(at least) trains an hour.
With the inherent advantages rail has (or should have) in this market there should be no space for the extensive cross Forth bus services that exist at present never mind hare-brained hovercraft schemes!
P.S The last time I conducted a "scientific" study of traffic was in July from a seat outside the Hawes Inn for a couple of hours having a few pints. I dont think I drank to much to affect my observational powers but it seemed to me the majority of crossing trains were 2 car!
I'm perfectly happy to let other contributors comment on the length of the average Fife train.
But bear in mind that you are comparing the Bathgate line (a half hourly DMU on a two track line, albeit a lot of services had more than two coaches) with the Forth Bridge (seven trains an hour, over a two track line)...
...there was plenty of scope to squeeze additional trains onto the Bathgate line, but there isn't on the Forth Bridge. Also, bear in mind that the Bathgate services are all running at the same speed (stopping at each station east of Bathgate), whereas the Fife services vary from "all stops" to "non stop between Haymarket and Leuchars", so its a lot harder to fit services in.
Because of the serious capacity problems over the Forth Bridge, I can see why the powers that be have considered Hovercraft/ additional bus/coach services to try to ease pressure from the trains.
Lastly, not everyone wants to go into central Edinburgh. There's demand for places in the west of the city (e.g. the 747 bus that links Fife to the Airport, RBS HQ, Gyle shops/ Herriot Watt etc) and demand between Fife and the north/east (e.g. a Hovercraft at Portobello could have a shuttle bus taking commuters to the offices in Leith or to the ERI). With so much demand between Fife and Edinburgh, a "one size" policy won't work.