What title do you hold after you've defended but before you graduate?

There isn't a formal, universally accepted title here. The general standard has been to call them "Doctor" since everything else is in principle a formality.

The only other note that I'd make is that it's not commencement that makes the student in question a doctor, but rather conferral of the degree by the university. Some universities may confer degrees multiple times per year, but only have one commencement ceremony.

So, I would not include "Ph.D." next to my name, but in the education of my CV, I'd write "Ph.D. (to be conferred Month Year)," as that is entirely accurate representation of the state of affairs.

Formally, they don't take a title before the university awards it, and the awarding is usually done by handing out a certificate stating the title (independent of whether that involves a ceremony or not).

In informal situations, it's nice to address them with the title because they are usually happy and proud of their accomplishment, and what remains to be done before they actually get the title is more or less a mere formality. But you shouldn't put any doctoral title on name tags, staff directories, or anything formal like that before they actually get the document.

In most countries "Dr" is an honorific (in Germany I believe it is a legal title and part of your name) that is typically used when referring to individuals that hold certain types of degrees (most doctorates but not all, for example holders of the JD degree are not typically called doctor, and some non doctorate degrees, for example a BMedSci in the UK). As it is an honorific I think it is perfectly reasonable to bestow that honor to someone during a party to celebrate a doctoral defence. Similarly when seeing someone the first time after the defence using doctor is a nice why of honoring them. In some countries one can pass the defence and still need to make substantial revisions. I would use the honorific as long as the person passed.

In non-formal situations I would avoid it, but then again I avoid using the honorific even for people with doctorates. In formal situations where honorifics are being used I would definitely avoid it. I think it would set a bad tone to go to a job talk an allow yourself to be introduced as doctor. In that situation I would deflect the honorific and say "not yet"