What mainhand mythics i should use for my hireling warrior axe? Arc and cosmicorb is my first choice but other also a good option,
Depends on your characters build in general. I’d try them all 
the hireling is run by the A.I., which you can adjust to how you want the Hireling to behave. that being said, I think you should do some testing to see which ones work for the Hireling while you are doing what you are doing with the Main. remember, what ever the Hireling does, even with the procs, you and your Main have to deal with it.
does anyone have some advice on how their Hirelings do with Mythic affixes in their MH/OH?
Earthquake or Cosmicorb
Rockblast is rubbish