No, that’s specifically for the Anniversary.
In your method, have you considered how the total bonus increases/decreases when adding one more weapon to the pool? For example, say that you have used 3 different weapons so far, and they are all perfectly balanced 1:1:1 (or, if you prefer 0.33:0.33:0.33). Now you pick up a 4th weapon, which you only get to use for a few seconds. Does your final bonus increase or decrease?
The total bonus must be monotonic with respect to the number of weapons used, otherwise you’re discouraging picking up new weapons (which is the opposite of what you’re trying to achieve).
We could calculate it in the Compare screen, I guess.
That’s known and the same class of glitch as so many others:
All weapons do.
Because it’s exactly the same class of message. It could be changed, but it’s messy and low-priority.
When talking about Extra Lives though, this is not “extra”. Your extra lives are zero in this case.
No
A large part of the database assumes that assignments are unique, so that is not an easy change.
I could delete all Key Rush mission histories (which would make them re-playable one more time), but that would throw the “Missions Completed” in the Information Overlay. You’d see numbers more than 100% 
Interesting idea, but it would only take a few hours before a list of all locations was made public each month, and everyone would swarm to them.