question

You should certainly keep the Lightning ground separate from all other ground connections. Giving the Multiplus it's own ground would be best practice. Personally I don't see fusing the negative pole of the battery to be any advantage. A direct strike is going to cause a lot of damage, however systems are earthed. Being in the mountains, you probably have thin soil over rock, which gives the high ground resistance. I would look at trying to create a ground grid for the lightning protection to lower the ground resistance. If you can do this successfully, then connecting the Multi plus system to this grid at one point only would be acceptable. The concept would be like turning the house into a Faraday cage, with all the electronics inside. This will include multiple grounds, a lot of heavy bonding straps using copper bar, and multiple lightning spikes to discharge the local field. I used to have a steel Sailing yacht with a 22m Aluminum mast. In 11 years, we had 4 close strikes - some which upset the electronics and compass. We never had a direct hit as the mast & shrouds gave a cone of protection. We only had one instance where one piece of electronics was actually destroyed by the discharge.

When you say this:

"The cabin is protected against lightning impacts with a lightning rod. Unfortunately, the resistance is a little high with about 100 Ohm."

My question is: A 100 Ohm resistance between the lightning rod and where?

I assume that is to somewhere else that you deem to be at earth potential.

Multiple earth spikes that are not directly connected will result in problems, not solve them.

Different earth spikes for different parts of the same building could be very dangerous during a lightning strike. They will develop high voltages between them as the lightning dissipates charge in the ground.

For a decent earth connection, you can use multiple localised electrically connected spikes to increase contact area. Pick your dampest ground and adding some salt will also help. But then everything goes back to that same earth.

If your property is surrounded by a metal fence make sure that is connected to the same earth, (on both sides of any gates).

Things that are likely to get hit, should go directly to the earth on an independent cable by the shortest route. Minimize bends and don't have loops in any earth cable.

Put a ferrite core around both positive and negative (together) of PV cable pairs coming from the roof. This will give your PV DC cables a high surge impedance and the discharge may choose an alternative route.

No fuse will provide lightning protection. It is too slow, and lightning has breached the air gap to the clouds, so another inch won't present a problem.

I think convention surge protection is probably too slow as well, but it won't hurt. I'd use them in conjunction with Transil diodes.

Use Transil diodes: battery positive to earth, battery negative to earth and positive to negative. Transil diodes have the fastest avalanche response I know of, and may offer some protection by diverting current to earth.

There will still be no guarantee.