Hi @ViveLEntenteCordiale,
I've had a quick look online and found this, which is worth a read:
www.thelocal.ch/20200317/explained-which-border-crossings-are-open-between-geneva-and-france
That said, it doesn't specify crossing the border for medical treatment. The French attestation allows you out for medical reasons, but crossing a border I don't know. The fact you're don't have a French CDS either is a problem I think (I don't either - was due to collect my new carte d'identité tomorrow but no chance!)
In your position, I'd be speaking to your doctors, your mairie, the Swiss authorities and even the British embassy. I'd also give the French embassy in Switzerland a go, explain the nature of your treatment and see if they can arrange an emergency residency/CDS type document that would get you back into France. They might at least know what to do for the best.
Is there a way your doctors can refer you to a French hospital for treatment? With all this uncertainty, I'd be trying to get treatment within French territory, no matter how far. If you risk going to Switzerland (assuming you can get in), you could be stuck there for the 30 days that the borders are closed. That might be worth considering, if you can do it and your treatment requires it. On the other hand, you need to check that they're not cancelling operations, as the UK is doing. Think about printing out a justificatif d'adresse that's less than 3 months old and with your name on, it's not ideal but it could help to prove you have a French address.
Regarding going out with your husband, I'm not sure of the facts but I would say have an attestation each and keep a metre apart. If you need to travel by car, sit with one of you in the back, furthest from the driver. If you have medical reasons that prevent you travelling alone, take some kind of proof. You know how much the French love their paperwork.
Best of luck - happy to do more digging if you need!