Apart from the Guru, do you think that a mediator is necessary in order to enable us to attain Self-realization?

Yes, yes. That is the Tutelary Deity or Ishta Devata. The mind cannot all at once rise beyond itself. The ego will seldom cut its own throat. Your consciousness will find it a hard task to realize the Cosmic Consciousness. Therefore the Name and the Form of a Deity are chosen to meditate upon. In due time, this Deity will manifest Itself before you and do the work of annihilating the ego and the mind, enabling you to realize Cosmic Consciousness.

Swami Sivananda