You don’t mix, you layer - Spray, wait till it dries down, then spray the next, when your layers are established, cover scent goes over it all.
All alc base can be sprayed on/over each other. There is debate on what order works best but you need to do your sweet spot testing first to find what works for you. Don’t spray any alc base or cover scent over A314 oil if you use it as it screws the diffusion.
Application points - just about everything is covered in this thread:
Personally, I suggest you don’t overthink them. So long as it is skin open to the air and with a good blood flow, all is good. 
For a single night, if you have your operative ratios, there is nothing to stop you spraying the appropriate amounts into an old togo, so you have your main application and an easy topup but its not a good idea in general for larger quantities.
One thing - IS has an operative life of about three hours per application. IO/Ammo are active for about twice that. Bear this in mind when you plan your topups.