attacking an eneamy from an ally country

There seems to be no change, albeit your idea about how the things are might not be 100% accurate.

There are a couple of ways around this. First, it's not the military access that's the problem (you get that automatically with an ally) but the alliance. So one way to handle it is to keep your jumping off point neutral with military access until you conquer your first enemy province beyond it. That province will become your territory, so you can then bring your friendly neutral into your alliance while using the newly conquered province as your new staging point.

Second, you can have your alliance as normal and use the 'acceptall' cheat to get your ally to transfer the first enemy province conquered to you. Then, again, you use this as your staging point and your away.

Additionally, you can use paratroopers to take the first bordering province then capture the rest of the territory from that location.

Very smart Mr_B. Never even considered that.