Germany is in this regard very similar to Italy, both for the population distribution and the late unification.

We know that fusion power works in the sun and in thermo nuclear bombs. Therefore, I think it is unlikely that it is impossible to make a practical fusion reactor. But it may need to be very large and very precise, requiring large R&D costs.

Compare these numbers to the annual costs for drilling / purchasing fossile fuels. And for the environmental damage they cause.

Unfortunately, the fusion research funding world-wide are just peanuts. The technology could be much closer to breakthrough if funding would have been better.

How is the separation done? Are these 2 different legal entities? Are they located in different buildings? Do they have different owners? Is there an overlap / exchange of employees?

The brand names are obviously not different.

The Big4 are quite active in consulting business at large financial institutions (at least in Germany). At the same time, there are only these 4 audit firms and auditors must change every few years. Thus, the likelihood that the same company is consulting and auditing the same client at the same time, or in short succession, is quite high.

The official "solution" to this potential conflict of interest is that 2 different sections of the big4 company do consulting and auditing. But I do not know how solid this separation is in practice.

And there is absolutely no conflict of interest when they do audit and consulting at the same time...

Yes, and also not the environmental damage.

Both renewables and fusion power can be produced locally and with much smaller environmental footprint.

This is not either-or but both renewables and fusion development must be funded sufficiently.

Both are much cheaper than what we currently pay for fossile fuels.