Curves adjustment tool

when working on the line in the curves tool (1), any change in one area result in massive changes in other areas

Agreed, however, one is often adding other points elsewhere on the curve to control colour or contrast, so this isn't a big deal in practice. Points are added where necessary, either to adjust other tones – or to "lock down" the curve.

(2). The pencil option is ugly and inaccurate

The "arbitrary map" curves created with the pencil tool can be very accurate and surgical in precision, however, they are often for special purposes and are not used in the same way.

(3). Could we generate a diagonal line divided into sections which will protect other areas of the histogram from changes in one zone?

By @Miron34664090c77b

Yes, either by saving a curve preset or by an action you can lock down different sections of the curve by adding control points. One can also introduce masks or blend if sliders to transition curve edits in a curves adjustment layer.

ACR / Lightroom has this.

Yes, it's a valid point. I often set a few anchor points to protect a straight segment prior to the adjustment.