If it was me, I would keep my funds in reserve for a solicitor and wait until the other avenues that I have suggested have been tried. The threat of a lawyer may be enough. Wait and see.
It’s good that you made a video. Now make the Etat des Lieux. You do not need to include all of the standard inventory form questions but itemize as much as possible.
She may not be mentally unstable but merely playing games that have worked for her in the past.
A landlord cannot demand repair/cleaning costs from you until they can demonstrate that they’ve actually paid out the money and have receipts. I would wait and see how far she takes it to get any monies back.
You now have the upper hand.
In the letter you write you should insist that she pays to bring the apt you currently occupy to a habitable state and enclose a list of associated repair/cleaning costs. These costs will likely be significantly in excess of the costs she is demanding from the previous apt.
Again, let me ask: did she provide quittances for the rent bank transfers? She is legally required to do so and if she hasn’t that will give you even more leverage.
Also, the new rent deposit should be lower than the previous deposit since the rent rate is lower. You should demand that she refund the difference.
In this letter you can write that you are “engaging the services” of Maitre Merouane Brahimi, 32 Rue de Marechal Joffre, Nice. This could be interpreted that you have actually hired him. He costs 150€ per hour and could write a letter to her for you, but again, I think you can hold off for a while on spending this.
Write your own letter to her and keep it simple and unemotional. Use bullet points to state your legal position.
Do not sign this new bail. Did you sign the previous bail?
I am guessing that you have not directly paid the tax d’habitation. If so, this weakens her even further. Are you directly paying the gaz/electrique, the phone?
You need to give her a couple of months to repair/replace items in your current apt. If she has not responded you can do this from the money you are saving by not paying the 800€ that she’s demanding.
Since you don’t have a signed bail then you might also be able to consider the current apt as unfurnished in which case you automatically have at least a 3 yr bail.
The current government guidelines on rental lengths of 6 - 12 yrs only relate to the Loi Pinel, which doesn’t strictly apply to your situation BUT it is the only guideline in existance beyond the 3 yr unfurnished rental rules and if you end up before a Tribunal judge it is certainly worth arguing that, absent a bail, the rental term should be at least 6 yrs.
Also, absent a signed bail and absent quittances there is no requirement for you to pay annual rent increases.
The Loi Pinel sets the rental rate in Nice at 12.49€ par metre carrez, loi carrez. That is why I had asked you how much you are being asked to pay and how much you have actually paid.
It is possible to speak with a lawyer for free by going to the courthouse/prefecture in the old town. I would imagine it’s a bit of a zoo and that the lawyers there will focus on trying to sign you up for their services. Lawyers here work for themselves, not for you but if you’re lucky you may get one who at least will provide a fair amount of work on your behalf.
I doubt you will ever need to go to court but you should create a file of all communications from/to your landlady. Keep in mind that anything you write can also be used in court by your landlady.
Again, try not to worry. This entire situation may work out well for you.