This is a terrible situation to find yourself in. As I see it you have two choices: first choice, you could lock horns with the landlord and fight to get the necessary repairs done (drains, condensation etc there seem to be lots of things to put right - how likely is the landlord to agree to fix everything?) then, if you are successful, endure the disruption of the work being done. How long would the whole process take?
Or second choice, move out.
I would be inclined to take the second option.
Thank you so much for your concern. Language is not an issue. Husband is dawdling because heās afraid that the landlord wonāt renew our lease in the event that we have nowhere to go. Landlord did this once when we sent him a registered letter. Said his daughter needed the apt. Then he calmed down. So we know he will do it again. I thought if we got our downstairs neighborās huissier employer involved it would strengthen the case our neighbor has. I suggested that husband read the thread. 
Maybe someone has some choice words of encouragement?
I believe that itās against the law to withhold rent?
Iām not sure who needs encouraging here. I understand that you may not want to move but this landlord has shown you who he is now. Twice.
In your situation, I would move. In the short term, maybe there are airbnbās that are empty this time of year and not too expensive.
Itās against the law to have sewage coming into the shower and air quality regarding ventilation. If they cannot be arsed to investigateā¦..
Thank you. Itās the next step. Gotta get husband on board.
Is this the same apartment you were having issues with in July ? What was the outcome of the window issue?
I appreciate itās not easy to find affordable places to rent but i would have been looking before now to move elsewhere. I wish you luck.
To All thank you for wonderful supportive answers
Iām at the mairie. Imagine that I was told that they cannot intervene and to go to a huissier. The landlord isnāt allowed to rent a place ie the second bedroom because of the sewage problem.
This might be just a language issue between American and English, but if you only āimagineā what the Mairie told you, instead of knowing what they told you, may I suggest that you go back there accompanied by a more fluent French speaker.
In the meantime I suggest that you start a search for an alternative place to live. There is no need for your current landlord to know that you are doing so.
As @Mike313 has said, do keep in mind the length of time that you would be inconvenienced by ongoing remedial work in your current home. Unfortunately, French workmen are notorious for starting a job with 1 or 2 days work (eventually), and then not coming back for 3 weeks or so. What should be just 4 or 5 continuous days of work can easily end up spread out over 2 or 3 months or more. Thereās a shining example just downstairs from you re the oven. You could still be fighting this battle this time next year.
Jeez, what is wrong with you? Of course the Mairie cannot do anything, and I doubt very much that they would tell you to go to a Hussier.
Adam, you may find this interestingā¦
Yes I did go to the mairie. And yes I was told to go to a huissier. I was directed to Proximite to advise me as to what to do. I have no reason to make that up. No need to be rude.
Thank you for confirming the huissier information. 
I thought youād given up? Or are perhaps having an exceptionally bad day? Iām hoping thats the reason for you replying like this rather than just that youāre incredibly rude.
Yep, that is me!
Before though you judge - consider the situation - you have poo popping up in the shower of an apartment you are renting and the landlord does not want to know. Like most here, you will panic and maybe you will post here a cry for help. Someone immediately replies that you should not hang around but report it to the Public Health Authority.
Now, how does it look if you just continue to waffle about [illegally] withholding rent with, I presume, the poo continue to pop up. So yes, of course, I am batting my head against a wall - (I wish there is that very useful animated emoji of someone batting their head against a wall in the emoji selection - it would be applicable to so much here..!)
Indulge me on this and continue: you have the poo in your shower tray situation for some weeks now and something inside tells you to speak to an authoruty. You choose your Maire. OK, it is a start. It goes like this:
you - knock knock.
Maire - āCome inā
You - āI am renting an apartment where poo is backing into the shower and the landlord ignores meā
now, insert here the reply that even the most unintelligent Maire would make:
a. āOh dear, pooor you. I tell you what, contact a Huissier who might arbitrate for you, and I hope your health holds up in the meantimeā
or
b. āOuch, this is serious, let me immediately call the ARSā (or whoever is in regional charge of health)
Well then since the mairie didnāt provide the name of this health authority that youāre referring to, can you provide the name of it? I called insalubritĆ© before the new year. The rep sent me a bunch of laws.
I find it very bizarre, itās considered a health hazard and then what?
This would be a good place to start; Signal Housing: to report signs of indecency of rented housing to government services | Service Public
Itās an odd term but sub-standard housing is called āIndecentā in France.
The above website has an online reporting form which will start the ball rolling to get the authorities involved.
Do you have Protection Juridique with your insurance? If so you can contact a lawyer for clarification, and if they send you to a hussier, any costs will likely be covered.