Making the best of a bad situation

Basically the master cylinder and booster don't know, nor care on what make of vehicle it's installed .
The very first thing to do is check/adjust the brakes, to eliminate the obvious.
The pedal return spring is interesting, and not supposed to be there.
I would be looking to see if it's there cuz the pedal is binding at the m/c pushrod, or pivot, if not, does it just pull the pedal back creating a clearance as you suspect.
If there's a gap created between the pushrod and booster, you need to correct that with the nut you mentioned, or as nec.
The other place for a gap is between the m/c and booster.
Just cuz it's an oddity, I'd gently separate the m/c from booster to check that gap, there is often an adjustment on the booster pushrod. Use something like plastercene, or mechanics dum-dum,
dead chewing gum?, to check .
If you take the spring off under the dash, and the pedal drops, find and repair that gap, - the pedal should seem awfully high at rest as it is now.
That's just what I would do, with what you got, - but as always, safety is the utmost concern, does the emergency work ?
Good luck