Skil 1825 router collet chuck tighten

[JFII]

I have an old skill 1825 router where a few months ago I put a bit in it to do some work but never got back to it. When I tried to take the bit out I could not turn the collet nut to loosen.

Then the collet lock started to slip so I ground down a regular wrench to fit and that started to slip. but the nut still would not budge. Finally I put the router on its side and supported the collet nut with some wood block then gave each side of the nut a whack with a hammer. This loosened up the nut and the armature shaft. I am a bit confused by the terminology  (see image) and would think the armature shaft is what the collet screws onto.  There are no obvious cracks or splits in any of the parts. I figure if I did crack or split something it would open up when I tightened the parts down. Anyway it is threaded for both the collet nut and the motor shaft, which is also threaded for it (at first i did think i broke something) My question is how do I tighten it (collet) onto the motor shaft. I'm thinking i need to disassemble the router further but what can I grip to keep the motor shaft (armature shaft?) locked and from spinning as i tighten the collet to it.  I also thought of sticking a screw driver through the case into the fan but I did not want to bend anything considering it spins at 25k or so and thought it much wiser to ask before I have shrapnel everywhere including me.

[wtnhighlander]

So, for this router, the motor's armature shaft is bored, slotted, and threaded, to act as the inner clamping component of the collet. Is that correct? And the flats below the threaded part of the shaft have become rounded, so that you can not hold it with a wrench to tighten the nut?

My first thought is that purchasing a new router is the safest path. However, it may be possible to carefully file the flats into shape, to fit a slightly smaller wrence. Also, very carefully clean and lubricate the threads of the shaft and nut before you assemble them.

[JFII]

More detail

What I did was lay the collet nut on a piece of 2"x4" so it was slightly supporting the router and gave 3 sides a whack with a hammer. Nothing. So I went a little harder after the 4th or 5th whack it was loose. I found the no damage to any of the parts.
I took the router apart. taking pictures as i opened it. All the face screws need to come out and only the bottom screws that go to the top side of the cover (I took them all out ) be careful of the small piece and spring that are part of the lock on mechanism. it will jump out and hide if you let it. I blew out with compressed air. I checked the brushes and they were fine. (did not disassemble the brush mounts i used a magnifying glass to inspect them)
I could not find anywhere on the armature shaft the had any place for a wrench so i reassembled the router.
Before I put the collet and nut back on i dialed down the speed to its slowest plugged it and and pulled the trigger. It seemed to spin up fine so i slowly dialed up the speed and there weren't any vibrations.
I unplugged the router and screwed on the collet and shoved a long thin screwdriver onto the fan blades and tightened. Here i was making a guess that when the collet nut is tightened onto a bit it it also tightens the collet onto the shaft. reasoning - The threads for the shaft were under the threads for the collet nut. When a bit bottoms out it is bottoming out on the shaft. I put the collet nut on LOOSLY and mounted the router into its table (I did this in case i had cracked the collet nut and when I spun it up it was going to fail an throw itself and the bit free the table would offer some protection) put a small 1/2" bit in and tightened the collet with its its wrench holding the collet in place with the wrench i had made when trying to remove it (I did not put the built in lock back on because i had damaged the hole where the screw goes to hole down the lock ) turned on the router at its slowest slowly turning up the speed - no vibrations so i cut about 3 feet of of scrap. all seemed fine.
I left the bit in and turned out the lights - just kidding.

Pictures attached what the skill diagram should look like and what it actually looks like