Namaste Phyllis,
thank you for the post and the interesting questions.
Phyllis Sidhe_Uaine said:

question #1: What defines a "sentient being"? I have heard the phrase numerous times but what is the "cutoff point" concerning who/what is sentient and who/what is not?
excellent question since this is emphasized quite a bit in the Suttas/Sutras.
for all intents and purposes a sentient being is a being which is possessed of consciousness, however, in our usage of the term, we mean to indicate humans, animals, insects and so forth. basically, any creature with a higher cognitive function. so, something like a bacteria isn't considered sentient whereas an ant is.
in a general sense, the term is meant to be inclusive of the Buddhist cosmology which has 6 forms of sentient being, namely, gods, asuras (sort of like Greek Titans) humans, animals, hungry ghosts and hell beings.
of course, these are my own views predicated on my very superficial understanding of the Dharma.

question #2: Does the "initial" empowerment have to be formal?
i'll break this one up a bit.
well... yes, in a nutshell.
I don't mean "Send $$$ to Bogus PseudoBuddhist Priest and get Instant Empowerment," or anything like with one of bb's banes, the Kaballah Center, but more along the lines of some of the ecclectic neopagans who are "self-initiates". (My sincerest apologies for the use of outside terms, but I'm at a loss for words to describe what I'm trying to say within your particular spiritualities, so I'm using words that I have a grasp of, albeit tenuous.)
this aspect of your question is quite interesting. as you may know, in the general hierarchy of Buddhism, there are monasitcs and lay people. however, what you may not know is that there is another sort of being, what we call a Solitary Realizer, a
Pratyekabuddha.
when we talk about Buddhism, there are several different ways we can go about it.. we can talk about schools, like Zen or T'ien T'ai, or we can talk about the overall Vehicles, like Hinyana or Mahayana and we can also talk about the individual vehicles, the Yanas, which a being practices.
in the invidual vehicle conceptions, we find the Pratyekabuddha. these beings are, for all intents and purposes, engaging in the process of Awakening without the benefit of a teacher.

question #3: I believe that, from what I've read between the two of you, one can have the "empowerment" without the Vow of Bodhisattva, but can the reverse be true; i.e. you can take the Vow of Bodhisattva without going through the "empowerment"?
well... in short, yes, though it isn't often the situation since this process of formally taking the Bodhisattva Vow is, in and of itself, an empowerment. not of the same sort as one of the Tantric empowerments, to be sure, but an empowerment nonetheless.
i, too, am a student and am by no means a teacher! if anything i say makes sense to you, it is due to your own good karma ripening!
metta,
~v