I take G in the picture and #4 in the exploded diagram to be the same part.
The diagram shows an armature fan (painted green in the image below) at the drive end of #9 - the Armature Assembly. I'm assuming that's what you're interpreting as "the standard gear that is in the exploded diagram"?
In any case, pry out the oil seal #1 visible in your 2nd and 3rd "here's where i'm stuck" pictures. Then remove the retaining ring #2 and you'll probably be able to remove G / #4 at that point. If not you'll have to remove the bearing #5 and the retaining ring #6 behind it, which could require a special tool, but lets cross that bridge if we come to it.
As to the "welded bolt and nut" end of the job. Haul it to a shop (or a friend with pneumatic tools) and have them see if a 1/2 drive impact gun and socket will knock it loose. Shouldn't cost more than $5 - might not charge you anything if they don't have to wrestle the motor out of the trunk of your car.
If an impact gun won't knock it loose, the weld will have to be cut away with a torch or die grinder. (I'd favor the die grinder - less likely to damage the armature.) Then try the impact gun again.