Rule 3 would be annoying, if you swap to spade to dig even 1 block all the marks are gone? How do you propose marking works? Look at a player and press mouse button? Perhaps use scroll wheel to choose what kind of marker to place.
Why not incorporate marking any location (build markers etc).
I added rule three so that people won't switch to binoculars and mark enemies just so they could snipe them alone. You need another partner to snipe for you. (One's a rangefinder while the other one's a sniper)