Should a cross-threaded (but functional) freehub body be considered safe?
Assuming you refitted the lockring correctly, and the threads are now mated up correctly, then mechanically it should be fine.

You should definitely use a torque wrench to make sure its tight enough but not too tight - a well calibrated elbow just isn't good enough for aluminium where the tolerances have been reduced by munching up a turn of thread.

Long term, don't be swapping cassettes unless you have to.  The freehub body might have half the number of refits left that it would have had if undamaged.  But it might be a 90% reduction or a 10% reduction too, depending exactly how much is damaged.

Its possible to "chase" threads to help move material out of the valleys, but it won't restore that material into the lands (high bits of the thread)  Plus a tap that big is uncommon.  Your LBS might have one, but you wouldn't buy one as a home mechanic.