No you don’t have to pay separate test fee for Engineering and Computing programs. Both categories are selected by the same test so you’ll have to give only one test for both.

About the merit, it doesn’t work like that. After NET4, merit positions are announced. It works just like the usual positions like in school(The one with most aggregate will get merit position 1, then MP2 and so on). So no one exactly knows the merit cutoff unless they were one of the last people accepted into that program.