-if I remove the seat post an pour/slosh something down it, would it get to the other tubes or just seat tube?
You would have to fill it up entirely.to get all inner surfaces covered .
IDK about you, to turn a whole bike upside down and rotated every which way takes a stronger person than I,
there are usually holes in the top of the head and seat tube to access the inside of the top tube ,
the BB shell to downtube connection is open at the bottom , and another hole in the lower end of the head tube.
some small vent holes are often there to let air out as it is heated in welding.brazing.
does framesaver fight rust on the outside?
No that is why it's painted .