Whether it needs to be serviced is debatable. Given that it is now out of guarantee you have nothing to lose by changing oil and filter. As I am fond of saying, nobody suffered a gearbox failure by changing the oil too often.
I am not convinced. The purpose of illuminating every warning lamp initially is to test them and ensure they are working. (It is known in industry as a lamp test function.) If a genuine oil pressure was ever detected how would you know it if the lamp does not work?
Good luck Davie. You deserve it for the effort you put into investigating the fault. I seem to recall reading that crank angle sensor is a common fault.
Bear in mind that remapping might be counted as a modification as far as your insurer is concerned, particularly if the power is increased, so unless you declare the modification the policy may be invalid.