Carbon assembly paste works great, even on aluminum to aluminum connections. I use it on my seatpost, on the seat rails, on the stem-steerer and stem-handlebars. I need less torque to make a secure clamp.

I don't think a different stem would make much difference. Your test of moving the stem on the steerer with a lot of sideways force on the bars is way more force than you'd have when riding. Much more typical is riding on the hoods, hitting a big bump, and having the bars rotate downward. And even that isn't likely to cause a crash.

If the stem-steerer is a little loose, there's a slight chance it could gradually move up a tiny bit, loosening the headset. The symptom is feeling the headset bearings knocking a little if you hold the front brake and rock the bike forward while standing over the top tube. So check that occasionally at the start of a ride.

That concept stem looks like a typical 4-bolt:


You shouldn't have to crank down on the bolts anywhere near enough to break them. Maybe the bolt was defective? For reasonable tightening, Torque Keys are preset to a specific torque, usually 5nm. Or see how tight the LBS sets the stem after the carbon paste is applied.