In general terms clutch disengagement issues are usually noticed when trying to get first or reverse gear from a stand still, whether due to a faulty clutch assembly or the hydraulics operating it.
Things like clutch slip are usually noticed in the beginning when in the higher gears, pulling hard say from fourth to fifth etc.
What you initially described of grinding/crashing when changing down at speed, I would describe as worn synchromesh cones (the brass clutches that match the gears to the synchromesh hubs in the gearbox itself) where if you change gear slowly they will engage quietly with no problem until really worn. This can be aggravated by low gear oil, high mileage or hard driving in general terms.
Tied in with your later statements of rattling when idling (possibly first motion shaft bearings or lay gear bearings etc) I suspect your gearbox is due for a total overhaul or replacement unfortunately.
With regards to your suspicions regarding the recent clutch job, some "fast fit" type clutch specialists have ways of separating the bellhousing from the engine enough to change the clutch without removing driveshafts or disturbing suspension (not something I would advocate) but may account for less evidence of a normal strip down. However on close inspection I would still expect to see signs of socket marks etc on bellhousing bolts and other objects around there.
Others may say different, I am basing this on my working experience since 1969
