Yes, I think a 2x2 embark would be the way to go; just to lesson the tedium a bit. As Girlinhat points out, channeling doesn't really impact path-ing performance.
I agree that dealing effectively with item clutter will be the major challenge.
Bonus points for doing this without using an atom-smasher.
1) By using magma to destroy most things
2) By maintaining contact throughout the game with caravans.
{imagines the dwarven broker in this obsessive compulsive fort}
"NO! I must have that particular cloth rope. I'll give you all this...
{waves his short little drunken arms at a quantum stack of stone crafts}
...for it. Our stupid stone crafter won't stop making this crap.
{stops imagining}
I don't know if this will work for all the stuff, due to caravan weight limitations.
Here's how I imagine this working.
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All the way down to one 'z' above the magma sea. With any luck a miner could blow right through all the cavern layers without having to fight critters.
At the end you have this:
...
.x.
...
and a huge pile of crapola that needs to be sorted into magma safe and stuff that is destroyable by magma. I plan on making extensive use of mining carts and quantum stockpile technology. (Using mining carts to create one huge quantum stockpile should also cut down on item clutter lag.)
Once that is done your miners drop everything not made from magma safe material into the magma sea. The rest gets turned into the lightest weight craft goods you can make. Maybe coins for metal and mugs for stone. Once you process the entire take from one stairway you collapse the stairway itself into the magma sea. (Except for the magma safe parts of the stairway.)
Complete one of those shafts and things will get easier because you can have your dwarves toss all the magma vulnerable crap down the hole via track stops as they collect it from additional vertical shafts.
At the bottom of the world you obsidianize the magma sea and then either engrave it, thus fulfilling the process every tile requirement; or you dig it up and make obsidian mugs with it to trade away for a spool of thread.
At the end I envision a mad mayor pulling a final level collapsing the final remaining structure and dumping the final material into a single tile of magma. I think you could do this and have just a single block of obsidian remaining from an entire embark.
Let me know if you can find any gaping holes in my plan, please. Or can think of any improvements that will increase world destruction efficiency.
I'd like to give this a try.