Checked helical gear it was properly torqued and washer look aswell.Attached are photos of both washers om either side of 1st gear on outputshaftFrom reading this am I correct in assuming your output shaft came out with the lower case when you lifted it? If so thats when the fork dropped onto the fifth gear input. Now if I suspect if you would have insured the shaft stayed in the upper case all forks would've stayed in place . The does not seem to be riding on the gear in your video can you confirm?
Also what does the inside diameter of the first thrust washer on the output shaft look like? It between the front roller bearing and first gear output. I want to see if its sharp like a knife edge or square like the outside diameter of the thrust washer. Also check the nut on the helical gear output to insure it is still torqued.