Hello Peter, and thank you for your time and your positive alternative!
Meta Box is like Excel... Many of us are using 5% of the options 🙂
Anyway, your solution did work very well.
I just had to change a few things into my CSS files, to get back my design on the form.
Indeed, I could keep IDs in the CSS, because the 2 IDs in a short code | #id-1, #id-2 doesn't work.
(well, maybe there is a way, but I'm not aware of it lol)
For people who are facing the same issue...
As the validation Checkbox was just for making people confirm them data (legal), I only ask a "Required" option, and I did switch-off the "Save field value" in the Advanced tab (because I have forms' data all clean in a "Save data in a custom table").