Except that 9 months took place on a space station. There were regular cargo missions to the station. And they could have been brought back at any nearly any point if necessary. Other astronauts literally went up and came back from the Station in that 9 months.

The timeframe being so long was almost entirely about the Starliner itself and what they were going to do with a known defective and potentially unusable spacecraft, where the only trained pilots were those astronauts, not anything with the astronauts themselves.

If the station wasn't an option for whatever reason (despite it literally being part of the planned mission), then other contingencies would have been available or at least planned already. This wasn't an Apollo 13 situation where not making it back was a serious concern.

To be fair, at the time, there was no ISS for the shuttle to dock to, the shuttle pretty much was all they had. It was designed for missions of about 10 days, and could be expanded to about 17 days if needed. If they needed to stretch it up to a month to go beyond that for her to have a second period, I suspect that would rather have used that cargo capacity for some extra food and such and dealt with her free-bleeding, and much beyond that they'd need to come down one way or another or just die in space.

It was almost half a century ago

They last 4-8 hours. Most women bleed for 3-7 days. So on the outer edge, you could need 42. I've never gone through more than a box of 24 in a cycle. But the US hadn't put a menstruating person in space before, who knew if being in space would somehow unleash a geyser of mysterious lady fluids never before seen by man.