how critical is passive stability?

Don't know about "tumbling". Do know about
"precession" and that comes about when a rotating
object is subject to a force at right angles to its
plane of rotation, ie, gravity. In free fall there is
no such force.
At any rate, there are several ways to ensure
stability in free fall of a rotating cylinder:

1) Have two cylinders counter rotating about a common
axis. Precessional forces should than counter balance.

2) Place your rotating cylinders inside a massive
shell, connect them to the shell via magnetic bearings
so that the external, non rotating shield can take up
any precessional load w/o itself being subject to
rotation.

I'm sure there are even better ideas out there, but
this is top of the head stuff,,,

GAry 7