1. It is not required. If you do place one, it can be either invasion or occupation, it's up to you! I personally prefer occupation, but if you try and don't think BLUFOR are being aggressive enough, go ahead and switch it to invasion and see if it suits your needs better.

2. There needs to be at least one objective under an OPCOM's knowledge at the start of a mission, it does not matter whether it is a military objective, civilian objective, or even custom objective.

Although I prefer to place military and civilian objective modules for the non-insurgent side in my insurgency missions (which obviously requires the Military AI Commander), I've seen other people only set up the Commander on the Assymetric side. I guess it depends on the type of mission you're going for.

I do this because I keep my playable units small. But if you're prepared to take a more 'you against the world' approach and give yourself the resources you need, you definitely can skip having an OPCOM on your side.

I like being a cog in a greater wheel so I place two OPCOMs.

Regarding whether you should be invasion or occupation, again that's really a matter of preference.