go with the concentrate! i have no problem just sweetening in the keg and leaving it cold. i usually use s05 yeast and it's pretty sleepy at fridge temps. if it is a tiny bit active, it's not quick enough, as the keg tends to go pretty quickly. but i guess that depends on what yeast you used.
i share your bottle bomb concerns, when i bottle from the keg, i know they are fine for a few days, as there is very little yeast in there, but if i'm going to keep them more than a couple days i simply pasteurize the bottles. this is far safer and easier than trying to pasteurize bottle-carbing bottles which can have an insane amount of pressure, yours will already be at a gentle equilibrium and there is no chance of explosions. i give them 20 minutes at 75 deg, and they are stable.