3) I believe that if you define a "STR" in an additional ruleset, it completely overwrites and overrules the previous "STR". so yes, if you re-define the "STR_PISTOL", the older "STR_PISTOL" information is gone. Everything inside needs to be defined again.
4) This is also answered by the above statement... if you want to redefine "STR_MOTION_SCANNER" to be something that does not need the research requirement, you simply add a new "STR_MOTION_SCANNER" into your additional ruleset file where the requirement is simply
not defined.
as for 2), I think the UFOpedia
screen can only handle 3 ammo types - otherwise it exceeds the screen boundary, and this has simply not been defined... perhaps a scrollbar should be added here? I don't know if that is possible, though.
EDIT: I just checked 1), I think you have found an unnecessary duplicate
at least I don't know why it would need to be defined twice