Mechanics and Fixing Any Questions Answered

Everything else so far has removed with no bother, other than a particularly stubborn plastic gear ferrule that was wedged in the stop. There's no visible signs of corrosion, other than the seatpost bolt, even the exposed section of stem where it enters the steerer is actually completely clean, which is why I'm a bit baffled

Update. I cut the seatpost bolt off with an angle grinder, seatpost came out no bother, actually greased quite well.

The stem has been a bit trickier. I got it moving about a quarter turn, but not up or down no matter what I tried. The face of it snapped when pulling on the bars, and it only moved using a scaffold pole on the remaining stump. Cut the stem off with angle grinder, decided to dissolve out the remaining lump with caustic soda. Now, I've never actually tried this before, but I got all my safety goggles sorted, gloves, set up a rig in the garden to make sure it didn't fall over, poured the caustic soda solution in, and it instantly fizzed and bubbled like nothing else. Within about 20mins almost all the stem was gone. Awesome.

Except what I hadn't noticed was the the bubbling solution had squirted up through the steerer into the crown of the fork, and it dissolved that too. So the fork is fucked, and I'm finding it hard to find a 650c rim brake fork anywhere. Great fun.