First of all, I don't really like the aggressive tone, given the pain that everyone takes to help you.
And then it seemed to me that the work should already be finished: in short, let's move on ...
The priority of priorities: and what to do before your expert comes. It is to uproot the arbutus that grows along the wall ... Because for sure, the roots will push the wall by swelling effect ... If this has not already been done, nevertheless it is the first thing the expert will see: and there he will take him for negligence.
So on the one hand it is something to do imperatively in view of the situation, and moreover, the expert risks using it TO NOT compensate you (if you know what I mean ... )
In addition, calling on a "free" expert appointed by your insurance is double-edged. Because indeed, these "work" for the insurance and on every occasion that it is possible AGAINST
"The insured". Which means that you will not be able to really take advantage of another risk or what do I know later - about this specific problem of your wall - since you will already be in their crosshairs (so in their database ... that they are exchanged between "insurance" companies, if need be, if by chance you change ...).
So it is just as imperative that the expert encrypts the work to be carried out, that you execute it (since if I understand correctly it will be you who will do the work) and that afterwards it will be advisable that you ABSOLUTELY bring the expert back to him show the work done, so that your insurance takes it into account, so that if by chance there was to be a problem, you would have proved that you would have done
"Everything we could ask of you in terms of" risk management "", and then in a second case, it would be covered by insurance, which would have no choice but to cover a possible loss: and above all you could sleep peacefully.
Now let us come to the problem of money, which you have raised more than once. Neither me nor anyone in this forum are not concerned by your financial situation - we simply make you attentive to the technical questions relating to the problem - and there is not a problem of money (as already explained, but really it is you who do not understand). The problem is, no matter how much it costs,
these are works that are to be done, because not to execute them would put forward
RT civil liability, if there were to be victims in the event of an accident, and it would be deemed that the investigation would demonstrate that you had not done the work required by the situation. And believe me or not, but if it were to go to court in a sad situation like this, the judge would not care that your income is modest: it's really not their problem. So if I shake you with that, it is so that you become aware of the real angle of attack of the problem, and not that you come to tell us about your personal situation to which WE ARE ALL SENSITIVE that you know it well (then even though we are very sorry not to be able to do anything individually, we all have our concerns ...).
Last point, if you want to do the work by roof itself, after removing the arbustre, you will absolutely have to remove all the mosses that we see on the photos, because it is also one of the possible causes that has damaged to the hanging of the first repair. And you have to be careful with these lichens, because even if we think we have removed and scraped them all, there is still some microscopically left in the rock - and that repels whatever happens as soon as moisture arrives by capillarity, either from the ground or from the wall itself, waterlogged by the river itself - so the only way to get rid of it is to
abrade the area (the surface of the stones) with an angle grinder or whatever => just scraping is not enough! Even less losing money on defoamer, bleach or whatever - this life form resists EVERYTHING. It is one of the oldest forms of life on the planet, for billions of years (?) It has been able to develop all the strategies, even the most unimaginable, to seek to win
colonize territories ... It's almost indestructible, even with acid it comes back.
Then you have to dust it off and then treat it (it is from the algae family, so it is microscopic and it can start again even under concrete).
Finally, do some mixing and adhesion tests with the concrete you will be making. By not forgetting to put a sufficiently thick wire mesh there (which you will rust to obtain a better adhesion, if so ...) and which will play the role of a frame. Reinforcement that should not be forgotten to cover with the regulatory thickness of about 1 inch of cement at least (for the right mixture, see the post of Chatelot). Do tests before, especially application and mixing speed (hydraulic cement) because don't forget that the implementation - to keep it going - is a real job! Possibly goes to a construction site to see how they do it and asks for advice, it does not eat bread: and we always learn things ...