Thanks for the feedback and bug report! I'll stop 2 novas from merging if the gameover sequence has started in an upcoming version.
That should also prevents a player from starting with a supernova.
It kind of seems harder to go from lvl1 to lvl6 than from lvl6 to the end.
After quite some attempts I've made it to the end starting from lvl1. The final score did not match the recent score listed bottom left. Maybe due to a late bomb explosion.