Moving the point along projection of the edge I wanted to move the point along projection of the edge onto XY plane. And I don't have any idea how to do this. One way is to move point with locked Z and snapping to the edge, but it has two serious problems: the target edge also will be moving and the snapping will be applying to the any edge of the underlied highpoly mesh (are there way to exclude this mesh from snapping I also don't know, I didn't found it).
So can anybody help me with this task? And yeah, sorry for my bad English...
If I've understood you correctly, a quite fast way to this move is:

Set your Pivot to 'Active Element'
Select your moving vertex, and then your static vertex, so static is active.
use S, ShiftZ to scale the moving vertex to the static vertex in XY.

But a better way (among many) to retopologize, if that's what you are doing,  would be to construct your low-poly mesh edges in plan view, in a plane above the terrain, without regard to the height of the vertices.
Then when you are happy with the XY form, from top orthographic, set Snap to 'Face' and 'Project Individual Elements', and hit G. The entire low-poly surface will then snap down, conforming to the high-poly mesh below.