1: splash CAN splash up to 10 tiles away (source JC) BUT as Meow said at 5 tiles away the chances are like 1.5%
2: splash goes threw walls... so it is 10 spaces with equal chance with or with out walls on those tiles.
as an added note:
don't just do an outer ring and inner ring of walls.... while plank and brick take a long time to get threw... once some one is threw all they have to do is run in and out destroying the braziers.
make cells with in your base so that each set of braziers has it's own walls protecting them....
as well as any border stones you have since they are now destructible. :'( </3