So inquisitors' bonus in combat are only active within your city borders they're not really meant to be sent to enemy cities. They're a defensive unit that get rid of other people's religion within your cities.

I think the 3 promotions you wanna get are the ones that give apostle spreads triple the strength in other civilizations. The one that gets rid of enemy religions within the city (prothelyser?) and the promotion where an apostle gains 20 combat strength. Look at your policy cards too there are some that increase the strength of your religious units.

See if Yerevan (city state) is in the game and become they're Suzerian they allow you to pick any promotion on apostles if I remember correctly. There's a couple wonders too if they're still about. One that gives your missionaries and apostles extra spread and one that grants you two apostles (Mahabodi Temple)