Hi JMB,
"I think we have noticed when the score sometimes resets to zero. If a player's bullet/rocket/etc. hits an opponent while the player is down, the score gets reset to this newly gained score (e.g. 100 points with message "hit chain +100" still displayed). Anything previously earned is lost."
Did you also experienced this in the current version? Thanks to your comment I also found that out in the previous version, but it should be fixed now in 1.2.15. Now you should not gain chain bonus scores anymore if you are down with no lives left. (That gave you the new score after the score had been internally reset, because well, the game was over.)
Most stars do stack in fact, except that picking up a double weapon replaces the shield and vice versa. It is intended however that you lose everything if you lose one life. This way you would be on your toes, careful to not get hit.
Anyway, I made sure that each weapon is just as viable without the bonusses and weapons have unique characteristics. Weapons with shorter range usually do more damage or are able to destroy buildings and the like. The only exception is the combat knife, which is for melee fans but does moderate damage to the bosses and is also the cheapest to buy. ;)