VERY Bizarre Combination of "Symptoms" I Believe Relating to Clutch or Transmission, or heaven forbid worse! Please help!
First, congrats on the most detailed problem description I have ever seen! Seriously, it does assist us.
All this is completely consistent with your clutch not releasing properly, meaning that the gearbox is still turning when the pedal is depressed. The syncros in the gearbox will then keep you out of gear, that is what they should do. You can indeed shift into higher gears however when driving as the gearbox revs more nearly match to what is needed. The jerking forward when you hit the starter is due to the poor old starter motor having to move the whole car instead of just the engine, because the clutch has not released !
Now as to why, it is likely that the clutch release fork has broken, inside the housing. I had this once with a Mazda, and symptoms were exactly as you describe. The gearbox has to be removed if this is the problem, and you should also then take the chance to renew the clutch, pressure and driven plate both, unless quite new.
Also a possible cause is a complete loss of clutch fluid if a hydraulic clutch, or a broken cable if that type of clutch. Loss of fluid in a hydraulic clutch can be easily checked in the fluid reservoir, and can occur if a hose splits
on Jun 20, 2016