Here's a tip to cover that situation. When you draw guidelines parallel to an edge that are supposed to lay "on face" start your guideline on the edge, move the cursor up/down a bit on the edge and then pull towards the direction you want. Nothing new here.
But, instead of leaving the cursor on face where you want the line to be, move the cursor to some alternate perpendicular (or so) edge on the same face. This forces the guide line to stay on face. At that point you can enter the offset distance in the VCB or move along that edge to the desired offset location.
To test if 2 perpendicular (or crossing non parallel) guidelines are actually intersecting, place the cursor on one GL and move it to the intersection. You should see a little red X. That indicates GL intersection. π
If all you see is a colored dot, the lines do not intersect. π The dot means the point at the cursor lines up axially with both GL's.
This works with GL's and edges as well.