If Kershner was directing the film, there would be proper pre-production and long shooting time. Look at ESB. They gave him something like 4 months and he took 9 months to film it, because that's what he discovered he needed.

But on the other hand, history would not repeat. ESB could only happen once, it had a specific time and place and neither people are the same as they were in 1979. Kershner is too old to fight for the film the way he did 30 years earlier. Lucas is too much of a dick to let Kershner get away it, the way he did back then (he learned his lesson)--and he's also experienced enough to know better. So Kershner would never in any realistic sense be hired by Lucas. Alternate history is never so simple as subbing in one guy for another--the entire context is different.