Always disconnect the earth first, no matter if it is a negative earth car or a positive earth car. This is the safe way to isolate the battery. The battery cut-off should be on the earth lead of the battery.
In my experience, having the clutch engage when the pedal is only an inch off the floor indicates a worn-out clutch. The clutch in my wife's F was acting the same as you are describing and I found the clutch worn down to just above the rivets.
Very odd, there should be no hard item under that spot. I just rebuilt a set of red leather Mk1 seats from a 1998 F and the backrest frame is just round tubing at the top with the foam backrest squab covering the frame.
Regarding the hand written numbers/letters, it should read PR3 because that was the project name for the MGF. We have one of the Fs from the first 200 batch and PR3 is written on many items, such as the backside of the door cards.
Having different size tires front and rear is normal for a mid or rear engine car, it has to do with the handling characteristics of a mid engine or rear engine car, which tend to oversteer and the tire size choice on the F/TF promotes understeer (safer). Yes, it was wisdom on MG's parts.
Actually, there are two bushes in the gearbox that the clutch shaft rotates on, UNM100010. I only replaced the top bush when I changed the clutch shaft, my access was extremely limited, only about 15cm between the gearbox and the engine, not enough room to get to the lower bush.
Attached is information about how the MGF Cup Cars reinforced the shock mounts. Mike Satur makes a kit for reinforcing the rear mounts https://www.mikesatur.co.uk/-MGF-Rear-Shock-Reinforcement-Bracket-Kit