I hope this forum accepts submission in English, I couldn't find anything about it in the rules . Otherwise let me know, and I can try to translate the post.
- I rented a flat in a big city on south of Germany for 800 Euros
- the renter was a friend of my girlfriend's mom, which is living in some other country since December, and considering moving there permanently
- she didn't want to ask the landlord for permission, because she feared having the request denied, so we just make an unofficial verbal agreement
- she just tell us the rent value, never mention any ancillary costs payments
- she tells me my girlfriend can move in with me, that it's no problem
-  given the Corona situation, she tell us that if things get worse, she might have to come back to Germany straight away, with no notice period, which we agree
-  given the flat was unused for so long, it was dirty, but we don't mention anything about it
- we move in on February 28th
- On the May 1st she tries to raise the rent price, saying we have to pay 30 Euros more for the additional costs
- on the message she sends me, she says it's an raise ("Unfortunately I have to raise the energy charges"), in my eyes also admitting this was never agreed before
- I say we're not paying, and ask her if we should move out because of the disagreement
-  she always fails to mention the heating costs we payed every month, both only used for a few weeks (meaning she would probably have to gives us money back instead)
- although she is bothered, she doesn't want us to move out, so she says it's ok if we keep paying 800 Euros
- on May 16th, I tell her I don't want to have any surprise extra charges, so I confirm with her the 800 Euros price
- we have some discussion, in which I mention the heating costs that she would have to probably refund us, but in the end she says "We should just stop this conversation. So just send me 800 and I am fine."
- on June 29th we find another flat for July, and say we're moving out
- she says we have to pay for 2 weeks (400 Euros), which is the usual notice period (which we agreed not to have )
- we say we're not paying
- she also tells us to do the handover with the neighbor, which we also didn't agree before, but we do it anyway
- She gives the neighbor some checklist, we go through everything, and she mentions no problem with the flat
- she keeps bothering us, and even calling my girlfriend's parents about the two weeks payment
- I block her from WhatsApp
-  she makes some sort of agreement with my girlfriend's parents, and says if we did clean the flat better, and pay the extra additional costs, Which she got from the energy company (110 euros) everything would be over
- although we got the flat dirty , we have cleaned the flat before moving out, so we deny both requests
- Again she fails to mention the heating costs , which we almost didn't use
- she keeps bothering my girlfriend, which gives in, and, without telling me, pays the 110 Euros and says she would clean the flat (I'm not sure if it was written or spoken)
- when my girlfriend tells me about it, I don't like that she did it, but I still help her clean the flat
- we thought things were over
- on the August 3rd, I got an email, saying she talked to some lawyer, and he told her that we still had to pay for the 2 weeks notice, implying she would take legal measures if we don't pay
- she is failing to honor  and 
- she also mentions some small damage to one furniture, which was never mentioned in the Übergabe, and we are sure it was either like this before, or the neighbor made it
Does she really have a case against us? Specially considering most of the agreements weren't made officially, we only having some WhatsApp messages, that, in my view, would strengthen my points.
This whole situation is being very stressful for me, so I want to know if I can also take some legal measure against her to put an end to this definitively?
If possible, just out of spite, I would want back either the heating costs we didn't use, or the 110 Euros my girlfriend paid, since our original agreement was only for 800 Euros.