from what ive learned there is a certain way models react to bones. depedning on the model the bones and wieghts can be in the exact same spot, but the model will react differently depends on where the edge points are.
i dont know exactly what you mean by the slightest movements get extreme results.
you should set bone limits. so an arm will only go from 180° to 90°. that way it cant hyper extend in any joint its not supposed too,
I havent used multiple objects to make a single figure. thinking about it doesnt make sense to me, you might want to take all of your multiple objects and make it 1 solid object. so to do that
make a new object.
copy and paste all your seperate objects into your new object.
put them all into place (ike your figure would look like,)
select them all and (build>join solid)
go to point edit mode
Select the "creases" (where your seperate objects would merge together)
hit ctlr+l(i think) or edit>merge points(i think)
merge the points togeteher so there are no creases.
then try to rig your model again.
this way you can add more points to where ever it doesnt look right. and i think it will clean up tour whole project since there is much less to deal with,
i hope i helped, and again, i dont know what you mean to begin with. i tried to cover all bases.