There are multiple distinct issues in your question, but I think the whole situation is worth adressing as one.
It seems like you are clearly being managed out. For context, in germany, it is not easy to fire an employee, and it is significantly harder to fire an employee with long tenure and no obvious brazen behavioral misgivings.
Thus managers often turn to issuing formal warnings for minor events or issues, in order to build a case to let someone go. This is often done to get rid of a long term employee, who is usually protected from most other reasons for firing them (for example, in the event of downsizing, the company generally has to give priority to people by tenure and other extenuating circumstances. If they justify that they need to shrink developer headcount, then they couldn't freely choose who to let go).
So it seems like you should be planning your exit from the company. Be aware that agreeing to resign in exchange for increased severance means you will most likely not recieve unemployment pay, which might factor into what you want to do.
To example, I've read about warnings, will they anyhow influence my
Warnings don't carry forward, and usually companies will scrupulously try to avoid mentioning negative aspects, even in a coded way in the qualitative reference letter, because using that to in any way sabotage your employee moving on is a good way to get sued in germany.
You can try and fight a complaint to get it stricken or invalidated, as far as I know, primarily at court. It doesn't seem worth it in your case, unless they go for the truly unethical move, which would be to try and let you go without any severance for cause, in which case I'd definitely fight it.
If a workplace has made up their mind that they no longer want you, then fighting in court will not change their mind. It's likely not going to be a fun or stress free place to work going forward after such an altercation. Following that, I'd only try and fight anything if they try to short you on your due severance.