I don't know what you are doing with your calculations monster however you appear to have mixed calendar years with financial years. The wage rises that had generally occurred on 1 Jan will move to 1 July so the "year" rises are measured in is now the financial one.
As for the difference between the back half of years 2 and 4, it is as follows. Year 2 in your calculations ends 31 Dec but you know already that there is no rise on 1 Jan (because they have shifted to mid year). It is a known 0. Year 4 however concludes and we do not know what the outcome from 1 Jan would be because you will be ready for a new EA.
Sure don't count your chickens but consider the possibilities. If the airline are still in the poo I would assume the company would want to freeze wages again. If they are making a motza, we will be pointing our finger at the wage freeze and demanding it be returned with interest (maybe a 7.5% claim). A fairer guess would be that another 3% rise would be offered then. If that was the case, for the 2018-2019 financial year would include a 4.5% wage increase.
Rather than complicate the matter confusing everyone, I will ask you to look at it differently. It is a 4 year deal, the first 18 months are written off as a wage freeze. The next 2.5 years you have increases of 3% pa (excluding compounding). 3% x 2.5 = 7.5%. You are to get 3% in 2016, 3% in 2017 and 1.5% in 2018. Added together it is 7.5%.