I don't think he was the only one in favour of a closer alliance and more steps towards a merger, at least on the French side. I believe some bloke from the French Ministry of Economy met with Nissan higher ups shortly before this all exploded with a plan for the next steps in integration. I've heard an observer surmise that the real issue is that the French government still has shares in Renault. Nissan has a reputation of being very close to the Japan government (it was picked to heavily invest into the Mandchoukouo at the behest of the policies helmed by Shinzo Abe's grandfather) but it's all unofficial. He is largely isolated though, for better or for worse he never played the French game of high end politics + economics, didn't take advice from ministries and had his own way of doing things.
I'm sure a lot of the accusations have truth to them, what's been out already (like using Versailles castle for his wedding and not raising an eyebrow to the renting being "free" or "gifted" by Renault funds) doesn't do much to dispel the image of slight megalomania* too used of ruling the whole thing as his personal fief. I'm sure Renault and Nissan discovering all those non accounted for / funneled funds right now is convenient but that doesn't have a bearing on the fact it was inappropriate and illegal.
Last I heard, the new Renault CEO has been very conciliatory and one of Nissan top execs reaffirmed the autonomy of brands within the alliance. So everything has been put on the backburner for some time.
* His wife wrote an op-ed in the WashPo and calls for the help of Trump that will meet Abe soon, for some reason (...well for the reason that French authorities are not willing to go beyond principle statements that rights of the accused have to be respected). Japanese justice is all around pretty harsh, but you know, it's not exactly endearing to try to curry political favor to ease judges in that sort of context.
