With full RPM on my rotor system, can I land my helicopter using only the throttle? That is, by just reducing power rather than reducing the blades' angle of attack?
Assuming you mean a manual thrust lever with out a constant speed regulator: yes that is possible but not optimal. Best is to keep the rotor blades at optimum speed and reduce AoA of the blades by lowering collective - if you do that the thrust needs to be adjusted otherwise the rotor overspeeds, but it is still better this way than reducing thrust/rotor speed first.
If rotor speed is slowed down first, the blade speed reduces and with the collective at the same setting, the blade AoA will increase. So the safety margin with stall AoA decreases, at a time when it is required most.
Possible - yes. Recommendable - no.
Collective only landing is not possible, Because there is forward velocity, and the flight will get out of trim as soon as the throttle and collective are changed.