At the beginning I would like to stipulate that I am not a lawyer and the knowledge about the rules I draw from public sources online.
as they claim "if you wait too long we can't guarantee that you'll get it".
They don't have any right to refuse the maternity leave. And everywhere I read it is clearly indicated that 14 days before giving birth is enough to notify the employer about it. Unfortunately I wasn't able to find specific law for this fact, but they just can't demand from your girlfriend such early notice about the date of commencement of maternity leave.
If I understood correctly, at the point of birth the mother receives 20 weeks maternity leave. 100% paid.
Everything is correct. One more thing: if she would like to take additionally the parental leave after maternity, she would get 80% of salary during maternity and parental leave (when apply for paternal leave max. up to 14 days after the birth), or 100% on maternity and next 60% of salary on parental leave (when applied later).
The parental leave may be taken for up to 26 weeks and can be share between mother and father.
one can basically go 6-5-4-3-2-1 month(s) before giving birth
Not months, just weeks - up to 6 weeks before giving birth.
Now, does this L4 only provide 80% of your salary like normally, or due to pregnancy one gets 100%?
It's still maternity leave, paid 100% (if no paternal leave)