Fill in the blank: AI, EO, IU, ___, UE

The missing letters in the series ab _ _ a _ _ b _ _ cb are:

In English alphabet, which is the 17th letter from your left?

If the first half of the English alphabet is written in reverse order, then which letter will be 15th from the right?

In a code language, if GREAT is written as 718222620 and MONK is written as 13121411, then how will VIGOROUS be written in the same language?

What should come in place of (?) in the given letter series? ACE, FGH, ?, PON