Now kcm fontconfig has subpixel rendering and hintstyle options depending on antialiasing. This means if I disable antialiasing, I can't set the other two options.

As far as I know, subpixel rendering and hintstyle don't depend on antialiasing and fontconfig does not prevent to set them when antialiasing is off. Indeed the previous version of kcm fontconfig had a different behavior, letting the user chose their values whether antialiasing was enabled or not. So I suggest to always enable them.

Then I'd like to get a clarification on DPI value. Reading the code I see that the spinbox accepts values from 24 with stepsize of 24.

QtControls.SpinBox {
                    id: dpiSpinBox
                    stepSize: 24
                    editable: true
                    enabled: dpiCheckBox.checked
                    value: kcm.fontsAASettings.dpi !== 0 ? kcm.fontsAASettings.dpi : 96
                    onValueModified: kcm.fontsAASettings.dpi = value
                    // to: need to divide to stepSize
                    to: 1008
                    // lowest vaue here can be == stepSize, that is because 0 means off
                    from: 24
                }

That seems weird to me. First of all, the user don't even know the spinbox accept only values multiple of 24. If it were this way, there should be a message to inform the user. But basing on information reported here, neither xdpyinfo, xrdb and fontconfig report values multiple of 24.

In example, xorg wrongly assumed that my monitor had 96 as DPI value, but I calculated it properly and the real value is 101 (see here how to do it). So I expect system settings to accept 101, but since stepsize is 24, it seems I can't set it. The comment in the section of code posted above says to divide by 24, but I can't understand why.

Maybe was this intended to format the value before it is passed to the system functions? If so, this should be done after the user wrote the value.