I, of course, believe Yoganandaji to be an avatar. However, it brings up several questions in my mind.
When Yogananda met Sri Yukteswar, he made him promise that he would reach God realization within this lifetime. But if he was an avatar, did he not already reach this state?
Also, after his guru died, he said that a blackness had polluted the river of bliss that had been flowing in him. How could this be so? I do not understand, though of course I still believe it to be so.
Those are good questions, ones that thoughtful devotees tend to stumble upon. We see similar instances in the lives of many great souls, avatars. So how can we reconcile their perfection in God with their humanity?
Well, I suspect in ways not unlike our own, just a difference of degree. As you can be noble and high minded in one instance, but perhaps irritable or frustrated by circumstances in another, and yet still be the same soul, so, too, an avatar who willingly assumes the garb and limitations of human form, must deal resopnsibily with the humanity in which he participates. Yogananda could be sweet or he could be strict, neither would necessarily affects his inward state of consciousness.
This does not mean, however, that the avatar acts from ego affirming desire, but that an avatar participates in normal and appropriate human feelings and interactions. Thus, for example, Lahiri Mahasaya, or Lord Krishna, were married and had children. This did not stem from personal desire but was appropriate to their divinely appointed role.
Often an avatar acts out their role (with sincerity, of course) in a way that sets an example or gives a spiritual teaching to others. Just so, Yogananda asks his guru if he will reveal God to him. It seems that in the lives of many avatars they go through a period of searching and have, sometimes, a moment (or period) of awakening to their higher state of consciousness. This is the sacrifice some make in returning to human form.
When Yogananda lost his mother, as a child, or later, his guru (in the example you give above), he felt the human sense of loss that is normal and appropriate. Yes, he could have accessed a non-dual state of transcendence, but such a one can play the role without being touched inwardly and does so, one imagines, for the benefit of others. In the case of grief and loss, inasmuch as the emotions are very real, we must own them before we can transcend them (rather than denying or suppressing them).
Nonetheless, I'll be the first to acknowledge that the ways of God are at times mysterious but the, upon reflection, is this not so for myself? For you?