Bank guarantee from the roofer due to defects in the roof

What should I imagine under such a guarantee? How is something like that set up and does it have any disadvantages for me? Will something like that appear under MY name in the Schufa? Unfortunately, I absolutely can't imagine what that means! Above all, what does our lending bank think about it? Does the bank even notice it?

With the guarantee, the roofer commits to contributing €2000 towards a repair within 5 years in case of damage. The guarantee has no disadvantages for you, as it is a unilateral declaration of commitment. Therefore, nothing about it appears in the Schufa – certainly nothing negative. The lending bank will have no objections to the guarantee itself. However, they do factor in the value of your house as collateral. This represents an increased risk if the storm clamps are not professionally installed. Whether the bank assesses that risk, I cannot tell you. Why I would not accept this deal: The costs in a damage scenario would be too unclear for me. The risk that there will be trouble with the insurance because I agreed to a non-professional execution would be too great for me – without having it legally clarified. There are 2 alternatives – additional costs unacceptable: 1. The roofer installs the storm clamps properly. 2. Another roofer installs the storm clamps properly, and the costs are deducted from the first roofer’s invoice. If your roofer “no longer feels like” working on the construction site and you no longer trust his work, invest a few euros to be able to supervise him during the execution of the work.

We brought in a friendly master roofer for assistance. Without him, as a layperson, I wouldn't have noticed the remaining defects at all. I am also somewhat disappointed in our construction management... But that's another matter.

It's not that no storm clips were installed. Only the correct laying pattern was not followed.
In the end, he is giving us a 2000€ discount on the invoice. And he is also guaranteeing another 2000€ during the warranty period.

A few years ago, storm clips were not standardly installed in any new building. That's why I don't see it as critically... It may also be that I am too gullible...

Now for a different question: How much does it cost to fix this defect? Is there some kind of ballpark figure? Because maybe option 2 from Hampshire is a possibility.

If the current roofer "just" doesn't feel like it, and is not completely incompetent, maybe the friend can help?

And additionally guarantees another €2000 for the warranty period.

Remember that it is not the roofer who guarantees, but his bank that issues the guarantee. It is like cash; for this, the roofer also has to pay his bank a guarantee commission. He should therefore be highly motivated to ensure that nothing happens during the warranty period. Otherwise, you go to the bank (not the roofer!), present the guarantee, and take €2000 with you.

A few years ago, storm clips were not standardly installed on new buildings

That is certainly not correct. As far as I know, these things have been mandatory since 2011 and were already strongly recommended before that. Our house, for example, was re-roofed in 2001 using storm clips, and we do not live in a "storm region."
There may certainly have been cases where the roofer did not feel like doing the job, or clueless/stingy builders voluntarily waived it, but given the dangers and liability risks of missing or inadequate storm clipping, hopefully those were exceptions.

I would also not accept the deal. Especially since the roofer will very likely still make real money from it in the end because he surely calculated more than the 2k for the execution.
I consider the guarantee pointless. If something happens during the warranty period, the roofer is liable anyway; the guarantee only protects against the company’s insolvency in the meantime. Considering the potential amount of damage (e.g., passersby injured, cars hit), the amount is a joke anyway.