Yeah, eerr, you err on many things. I will take the time to correct you on all of them.
b: flow can push creatures through fortifications anyway(magmamen/fire imps who investigate)
This is a misconception that only applies to creatures in underground water features, like cave crocodiles *shudder*, olmmen, lizardmen, and the rest. They do not get pushed through fortifications by the flow. They can freely path through them. This does not apply to creatures from magma features or any other creature; fortifications will safely prevent fire imps from entering your channel
d:fortifactions are covered by molten magma, so if they are there you can't see them without looking at the tile.
If you've set the init options to show fluid levels, then you can see where your fortifications are because there will be a dark red approximately equal sign there. Otherwise, yeah, you'd have to look.
e: when filling from a magma pool, i'm not sure if creatures respawn, but either way you need to dig low into the pool depending on how far you need the magma to go (16x16 and you'll probably want 5-10 layers of magma)
Magma creatures don't respawn. The three or four fire imps that came with the magma pipe are all the fire imps that will ever be on the map. And I'd advise against trying to fill a reservoir from a magma pool, as too much magma will be lost in the process. Much better to build your metalworks on the magma pool itself.
f: magma does normally flow upwards, there will be no flowing up out of the channel to cause flooding unless you use a screw pump above the channel.
Magma does not flow upwards. I've dug several magma channels that dropped a few z-levels and the magma never flowed into my forges. Magma flowing upwards only occurs when the magma is pumped, at which point it's tendency to flow upward is more a property of the pump itself than the magma.
h: you can create infinite magma by filling an enclosed 1x1 tile from a screw pump at the same level in the pump, both over the magma source.
good luck powering it without bauxite or adamantium, and also not flooding your fortress.
You can't make infinite magma using glitches, if you ever could. The infinite water bug was unique to the way water pressure is handled in the game, I believe.