I rode a probably similar japanese 10-speed as a commuter for a couple months that had a 3-bolt steel chainring and cranks with the same wimpy little bolts...threading directly into the chainring. I had it set up with only the small ring, ends of the bolts passing just a little through the back of the chainring. So one night i was riding along, downtown at night, and all of the sudden i hear a loud snap/click/metal sort of noise and feel something through the pedals. I pull off the road a few seconds later and see that one of the 3 chainring bolts snapped off at the head and the remaining two weak little arms of the spider had torqued themselves out of true when it happened. Pretty much the end of that bike, though i bent the spider back as best i could and used different bolts; it still rode for a few days before i realized it probably wasnt a good idea to have a ghetto singlespeed whose chain tension dramatically changed throughout the rotation of the cranks...and freaking cottered cranks are impossible to remove.

Anyways, i torqued those chainring bolts about the way you're describing you did, tight, but a little looser than i thought would make them snap...

I'd reccommend some new allen-bolts, probably look nicer and also give you some confidence about them. Granted, that bike lived outside and was thoroughly abused every day, its up to you.