I have a path consisting of three nodes where two nodes are in a 45° angle. Now I want to shorten the angled part and keep the angle.
Is there a way to snap two nodes and drag them across a specific angle?
If you drag a shape to different size with the normal selection tool and holding Ctrl at the same time the size changes proportionally. That means all angles stay intact.
I guess scaling the strokes is prevented in the preferences. If that's true you must set the stroke width temporarily to zero because in Inkscape the stroke width unfortunately is counted to object size and that ruins the proportionality. In Illustrator that does not happen.
Duplicate the existing line. If you have snap to path ON you can easily move the endpoint along the duplicate if you make it smaller. Delete the unnecessary version. If you try to make a line longer you can draw a new line (zero stroke width) move it or scale it proportionally longer, send it to back and drag the original endpoint along the line.
Learn to reach shapes and lines via the Objects panel. There you can easily for ex lock those parts which do not stand accidental edits and you can select parts inside groups and under clipping masks.