Yamaha FG 512

I've never had one that wouldn't loosen, but there's always a first!.  Try harder counterclockwise, remove it, and add a light grease to the threads.

The truss rod thread doesn't go thru the nut, so you can't lubricate it before attempting to remove it.  Heat might help, but you'll need to hold the soldering iron on it for a while to expand the nut.

You have to sand the heel to change the neck angle.  The fingerboard goes goes for the ride, it does nothing structurally.

Did you read my neck reset guide?  You're missing a lot of steps.

AH! You're in luck, the threads go thru. I've seen some that don't, but the one from this era does. I have one in front of me.

Did you read about calculating the amount to take off the heel to get the aprox. neck angle? It's really a guess since the neck will bend up some under string tension. AND shims need to be glued to the dovetail to take up the slack in the joint. That is very important! That is what holds the neck angle in place, along with the glue.

The notched straight edge is to contact the fretboard. I made some but rarely use them. I use a flat straightedge the contacts all the frets. The frets are the working surface, the fretboard surface means almost nothing. You need to adjust the neck relief until the neck is flat (across the frets) to get the proper neck projection. I've been looking for .07" above the top of the bridge neck projection lately. Beware it is very difficult to hold the neck heel flush to the body and the fretboard flush to the top to get this measurement. Check it many times. You'll probably find some variation depending on how you hold it.

The shim under the fretboard should have been the last step before gluing the neck back on, since you don't know the proper neck angle with sanding the heel. You've skipped many critical steps. I takes many hours to get the proper neck orientation. Doing it incorrectly may only be noticeable after gluing the neck back on. Then you will have to take the neck off and do it all again.

Another important thing:

I want to glue the dovetail neck in again.
Where should the glue go?
The whole room in the Swallowtail surely does not have to be filled out.
I have applied colored dots.

Which places need to be smeared with glue?
The red dotted surface under the tongue of the fingerboard surely.
But the other areas??

I´m sure somebody knows...

Thank you for your friendly answer!
You helped me a lot!
I´m going to turn on the baby bottles heater pot and prepare the hide glue.

Will it be necessary to brush glue also to the bluish dotted aerea?

Thanks so much for your help!
I´ve read your gluing advice only after I had done the work...
Furthermore I had made the shim and measured and all looked fine.
So I heated some hideglue and put the parts together.
Next morning when everything had cured I noticed that the shim had been a tad to high  .
When I tried to loosen the gluing I made the experience that my hide glue
was super strong. I pulled the 15th fret, heated the fingerboard´s tongue and tried to push a palette knife under it what was very difficult.
When when it finally worked I changed over to a very thin Japanese Saw and succeeded
in dividing top and fingerboard. I didn´t do anything to the heal but ground a tad off the shim until the strings didn´t buzz anymore.
Now im going to reglue both again - but this time with Titebond Original.

Btw. I took the pick guard off because it was in the way.
What glue would you suggest to reglue it again?

I add two photos to show what I did: