The two stage cascade is a very good idea, I'll use it for my design, as it needs a very low leakage when control voltage is off.

No, once the procedure is completed there is no more communication on I2C busses (the DACs stay at the same value until a new procedure is initiated). And there's nothing about this in the datasheet...

It seems very simple but I guess it won't work in my circuit, because a voltage divider pot is inserted between the input jack and the clamp circuit. If there is nothing wired to the input, or if the input jack is normalised to ground, the output value will not be 0V. But thank you for your idea, I keep it for another design !

Yes, sorry, it's the first time I ask for help on this site. Anyway, thank you for your answer !

Yes, you're right. Thank you for your help !