You have two distinct questions - you legal position on permenancy of employment, which I can't answer. On the mortgage salary cert, the most recent document is the one that the lender will use. They may or may not query the discrepancy on permenancy from the previous form, but you have to assume that they will.
Some lenders will accept that a similar role in a similar organistion counts as continuity of employment, where you have moved jobs for the advbancement of career, and you have already completed 8 months and have been happy to lend to, say, contract computer programmers on this basis.
The problem is that a lot of lending work is automated, and that forms are processed through a computer system that allows automatic passge to the next stage if all the boxes can be ticked, by say a clerical person.
If something goes outside the criteria, it might stil be passed but has to be referred to a more snior decision maker, who may act arbitrarily, or refer back to you for clarification.
As such, your progress will depend on which lender you've applied to and how good the rest vof your application looks.