The answer, in my opinion, to all the questions is "It depends."
"Healthy" is a very subjective thing. Some people wouldn't consider a poly relationship healthy at all, regardless of whether there's a commitment or not. For some people, making a commitment might feel unhealthy because of past experiences, or because they're only doing so to please their partner, or because it makes them feel trapped... etc.
Whether it's healthy to make a commitment depends on the people involved and the purpose behind the commitment. And what specific commitments are or are not healthy also depends on the people and the purpose.
Personally, I think any type of emotional connection with another *is* a form of commitment. If you're friends with someone, you're at least implicitly committing to spending time with that person, talking to them, enjoying being around them, etc. If you're in a relationship, whether poly or mono, you're at least implicitly committing to spending time with that person, respecting their boundaries, communicating with them, etc. This is how *I* see it, at least.
A commitment to someone else doesn't have to be a formal thing, and it can be whatever level and whatever type the people involved are comfortable with. Hubby and I are committed to each other by legal marriage, as well as by promises we've made to each other over the course of our relationship. We started off as mono; when we transitioned to an open marriage and then to our current arrangement, where I'm poly and he's exclusive with me, we refined those promises, eliminated some, and made new ones.
S2 went into this believing he wasn't ready for any kind of commitment, since his marriage only ended several months ago. His profile on the site we met through specifies that he's only interested in non-monogamy. But somewhere along the line he apparently changed his mind. He expressed his commitment to me by saying, "Well, so much for non-monogamy; seeing other people doesn't feel right to me" and by changing his relationship status on that site from "Separated" to "Attached." And I expressed mine to him by telling him I wanted to be with only him and Hubby, and by adding a line to my profile on that site stating that I'm both married and attached, and not seeking anyone or anything else.
So on top of whether a commitment is "healthy" or "unhealthy" being almost entirely dependent on the people involved, the way that commitment is expressed and defined can also vary widely.
Long story short, as I said at the beginning... it depends.