Can I land a helicopter by controlling only power, not 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.