I stripped one of four bolts on the faceplate of my stem. Is the stem usable until the replacement arrives? The aluminium stem on my road bike says the max torque for each of the four faceplate bolts is 6nm. So I set my calibrated torque wrench to 6nm... and one of the bolts stripped its thread.
I guess “max” torque is a true upper bound?
Anyway: with three remaining good, tight bolts, is my stem likely to be safe to use for a couple of days until a replacement stem arrives?
This really is an opinion more than a factual question.  That you still have three bolts means its held together, but it was designed with 4.
So there's a non-zero chance another bolt will go because the load is increased.  Look at the Arecibo cables just last week for an example of how quickly things can fail.
Nobody can categorically say &quot;that is safe&quot; OR &quot;that is unsafe&quot;
You're a functioning adult and can make a judgement for your own risk tolerance levels.  You know how much pressure it took to strip one thread, now that there is one-fewer holding the load the remaining extra to strip will be less.
OPINION: if you have another bike as an option, ride that instead.   If you can take a bus or walk, do that.   If you were stranded somewhere, you might choose to ride very carefully, or walk, or phone for a pickup.
I would not ride that bike until it is repaired.   You only have one face, and a stem failure can be catastrophic.