Legitimate Expenses for Irrevocable Trust

It's like with anything else - espenses that are ordinary and necessary to the business or investments of the trust can normally be charged to the trust. In part it will depend on exactly what the trust says, and in part what the law of your particular state says.

Yes, the kind of flexibility you want. It's all based on how the trust is drafted. If you can't find it in the trust as it is now, you won't be able to do it, unless you can convince a court to do it for you.

Can you give a few examples of the kind of flexibility about the size of the gift to each beneficiary that can be left in the hands of the trustee?

What are some limitations on that flexibility?

If you make explicit what you want in the terms of the trust, you can do pretty much whatever you want. However if a trustee has too much power to make gifts to himself [directly or indirectly], there might be an income tax problem. And there might be a gift tax problem. But as long as you're not rewarding illegal or antisocial behavior (e.g. stating that a beneficiary won't get anything until he divorces a wife you don't like), anything you make explicit is probably going to be OK.

Asking vague, open-ended questions in this regard is really unproductive. Entire books have been written on this kind of topic, and without specific facts to narrow it down, there really is no good way to answer your questions other than in the vagues terms.