Eric, bring the beaver to your sets.When there is lots of sign you could probably set 100 traps.What I do is make castor mounds ,I bring the beaver to where I want to catch them.As they are cruising the river they will smell the foreign castor and come in to see who the intruder is--don't be afraid to gang set.I alternate my lure at the sets.Give them a choice.My favorite trap to use is the 330.
In answer to your second question--yes beaver cross over one dam to the next.You should be able to see a main trail over the dam.Place a 330 on the bottom side of the slide ,I usually half submerge my 330's[check you regulations].Dam crossovers are also dynamite otter sets.
Good Luck. Tom