I think I saw something in the last photo that said 'No Sharp Blows'? There is one school of thought that says it's OK to loosen the comp nut with an impact wrench but not to tighten with an impact wrench. I don't use an impact but I guess some do. Point being, I would exhaust every other approach to removing the shaft extension before I started banging away on the output shaft with a hammer and cold chisel.
I have used a split puller like the one linked in Klarich's post to pull the inner race off the trans mainshaft when the race has walked into the 5th gear mainshaft seal and there was not enough room to get a standard race puller behind the race. Used a dremel to cut two notches in the side of the inner race deep enough for the clamshell of the split puller to get a bite, a little heat and she pops right off.
I would be trying that approach and cut notches in the shaft extension, heat it up a bit and try pullling it off before banging away on the output shaft and the case bearings and seals. Then heat up the remaining bit of the comp nut and back it off; various ways to get a tool on to turn the nut. But that's just me.