Let me see if I understand your basic concept (phrased assuming that it's possible):
Magma pipes fill themselves until they are full. By taking magma out of it, and then putting it back in, you can trick the pipe into thinking it's not full when it is, causing it to overflow.
I'm not an expert with magma pipes, but I believe the way they work is that they fill themselves until they reach a certain level, then stop.* If that's how they work, then your planned design will not work any better than your current design.
*I'm not sure what happens if you reshape the pipe. I suspect it will fill any shape, but larger shapes will fill more slowly (and conversely, smaller shapes will fill more quickly).
If your problem is that it flows in too slowly, then the best you can do is have some kind of large "magma storage facility" underground near the pipe which you pump out at the same time as it when you hit the "end the world" switch. It's twice as much work, and after the storage facility and the magma pipe are empty, you're stuck with the same speed of lava flow. But if that burst is enough to cover the world in lava, it'll probably be enough to destroy it sufficiently. If not, build a bigger storage facility! Use math to calculate how big its volume should be, if you are so inclined. If not, just guess.
Powering it:
I like perpetual motion machines. If you don't you're stuck with either a river, or massive amounts of windmills. And tons of axles.
Draining from the base of the pipe will take a lot more work, but you'll be able to use ALL the magma stored in the pipe, because you can only pump out the stuff that's immediately below you, and that is only filled by the stuff above it falling back down (hope that made sense). So when you're pumping at a certain level, you can only drain the pipe to the level immediately below it (because pumps pump from one level below). The rest will just sit there.
Framerate:
No idea. I think it'll hurt it a lot, but it may or may not be crippling.