I think a more appropriate idiom would be "in the thick of it", adapted here to be "until they're in the thick of it."

It communicates that you're in the middle of something chaotic/stressful like a jungle. The phrase is very natural for native speakers.

This phrase doesn't make sense at all stand alone. I couldn't find the context either.

Safe to say that no, not idiomatic. And if you use this phrase as quoted people will be very confused and have no idea what you're trying to communicate.

I think this is a typo. “Until they’re in it” is the correct phrasing.

"Until they're in it" totally is a phrase that makes sense and I would say can be idiomatic.

OP if you read my previous comment, then that does not apply to the phrase that ethanbond corrected.