Starting up the car with the clutch engaged and the gearbox in reverse was to check if the clutch disc was stuck to the flywheel or the input shaft was binding in the pilot bushing. It doesn’t sound like you have either issue.
The throw out bearing you installed two years ago had already worn down within 1,000 miles? Is that why you replaced the clutch slave – figuring it was not retracting the pivot arm properly? Did you do anything with the clutch master?
It sounds like the throw out bearing may be rubbing on the pressure plate even when the clutch is not engaged. Will the pivot arm move freely slightly before getting stiff resistance? Do you have a photo of the clutch slave and the pivot arm in the bell housing? I am wondering where the pivot arm is sitting ‘at rest’.
Are you sure you don’t have air in the clutch hydraulics? The bleed screw is on top on the clutch slave? Maybe try quickly pumping the clutch pedal to see if that makes a difference in being able to go into gear?
If the main bearings are bad, at idle you probably will see an RPM drop when you engage the clutch.