For those still wondering, you can do it all solo except the finale (vault door has a two person lock) and if you create an invite only session (If on PS4 start offline, hit start, hit online tab, hit play online , hit )invite only ) it's better so you don't have players tailing you.

Diamond Casino Heist Payout:R$10.34 -R$11.37 Million\n\n The Diamond Casino Heist payout is by far the highest in GTA Online. The rewards for the mission add up toR$10.34 million on normal difficulty, and it can go up toR$11.37 million, if the heist is on hard difficulty.