Okay, so if the lowest payslip was £1,500, UKVI will calculate that as £18,000 or £600 pounds short.
So you would need £16,000 + (2.5 * £600) = £17,500.
So you are okay on the income front. Whew!
It's the living together and the working while traveling that could seem a bit off to the ECO. Hopefully your lawyer has a proven track record with similar applications. I wouldn't worry until you have something to worry about. IF the application is refused, are you guys prepared to marry?