I hope the 'used' mask, machine, hose they gave you were only used a little bit during someone's trial period. It would be wrong to give you a hose or mask that someone wore for a month or more. I believe they can be sanitized. It makes sense that they wouldn't throw a away a machine because someone used it for less than 1 month during a trial, but I wouldn't like to find out I was given a machine that someone had been using for 1 year or more. In the United States, when I get a replacement mask or hose (e.g. every 6 months), it is a brand new one.
Maybe you could try living without using the water chamber for 7 days during the trial, but after that, if you are keeping the machine, it would be completely unreasonable for them to ask you not to use it.
Since you know that a good pressure for you is 8 to 20, I suggest setting your trial AirCurve Vauto machine to those Auto settings. Then if they call you and you have caller-ID and you can tell it is them, maybe don't answer their calls for a few days until you get a few days of better results from this machine. I hope they wouldn't change the settings back via the modem, that wouldn't be nice of them. I feel that if they do something like that, you'd want to go somewhere else for treatment anyway.
When you do talk with them again after a few days of good results, they will probably be angry that the settings were changed, and you could say 'I felt I couldn't get a breath and was starving for air, and when I told my friend, he showed me how to change the setting'. (Don't talk about the CPAP forum or finding a provider's manual, that will annoy them even more.) Then they will tell you that you should never ever do that and you can 'Oh I'm sorry'. I suggest talking with them yourself in broken Czech instead of your husband talking to them so you can purposely make it harder to communicate during that part of the conversation
. Maybe say the equivalent of 'My Czech poor. I not understand everything I hear. But is my sleep result OK? I sleep better now!" (It will be less likely them to yell at a person who seems not to understand.) But by then you will have proved what is a good setting for you so I doubt they would reduce the minimum pressure again after that. If they do (maybe out of spite), then you probably should go elsewhere for treatment.