A few remarks :
4. I highly doubt that Germany and Italy, even by pulling their resources together, have the technical know-how to construct a first line fighter by themselves, at least in a realistic time frame. Developing and constructing a first line fighter is one of the most difficult engineering task at the time (second only to space exploration). Not only one need to possess every brick of knowledge to make an airplane, but you also need to know how to make those parts work perfectly together. And what was the most advanced airplane that either country developed on their own, the Fiat G.91 15 year prior. The step is simply too big.
I doubt Germany plus Italy and he Netherlands lack the technical capacity to build a plane on their own and certainly given the projects they were running (AVS, VJ101, VAK191, Fiat G95) the governments in question didn't really think so. Still if it fails to work out the logical candidate is probably not F/A-18 but Northrop's F-18L. Regarding 1 or 2 engines the NKF design was certainly single engined. I suppose its possible that you go to something like Panavia 100 instead but wouldn't take it for granted.
On the engine(s), SNECMA is still sub-par and will only fully catch up with the M-88, but, not developing the M-53 will set back SNECMA by, a least, a decade.
So, to keep the French on board, you need to still give the lead for the air-frame to Dassault, ensure SNECMA get enough new competences to keep them happy and keep the possibility that the electronics are developed, partially, on a national basis like OTL Jaguar. So nothing complicated at all .
Having Marcel Dassault happy with the project is I suspect the key component... but the proper POD here is SNECMA in 1959 teaming up with Bristol instead of P&W. Instead of TF306 they are building an engine that start from the core of the Pegasus together with Bristol which is what ends up powering ACF/AFVG... and the Mirage F3/F3M that goes to complement it.