This is absolutely bat****. There are plenty of roads between Ely and King's Lynn.
No there are not. The other options if the A10 is closed are very lengthy diversions.
The road network in the Fens is very sparse because of the limited number of crossings over rivers and drains. Furthermore, many of the minor roads are not suitable for buses/coaches because the variable shrinkage of peat makes them difficult to maintain, with a risk of long wheelbase vehicles "bottoming out".
There has been a lot of rain in the Fens in September, and more importantly in the headwaters of the River Ouse. There is lots of flooding already, including the Welney route being impassable. This means that on the western side of the line there is nowhere to cross the Bedford Levels between Mepal and Downham Market.
On the eastern side of the line lengthy diversions are required to cross the Little Ouse and Wissey rivers, for a bus/coach that probably means going via Mildenhall and Thetford.
Google Maps will tell you there are ample routes to drive
For a 4x4 car maybe, for a bus/coach no. Does the flooding at Welney show up on Google Maps?
they could even advise customers to use their services to Peterborough and existing buses (Excel) to King's Lynn
The last buses Kings Lynn-Peterborough are 1905 and 2125, returning at 2040 and 2245.