1. The only pickup for Rocket Launcher on Staunton Island Phil's Army Surplus, where you can buy one, but only after "Arms Shortage", meaning that you HAVE TO complete "Paprazzi Purge" . 5 Rockets cost $ 25.000. Another pickup is at the hideout after you have collected all 100 Hidden Packages
2. There IS a "Trick" to passing Bling-Bling Scramble! You have to destroy the opponents' cars as soon as possible, and then just drive around collecting checkpoints.
I recommend stacking up on Rocket Launcher ammo first. Then start the Scramble and drive towards the end of the road to the first checkpoint. Then get out and shoot the cars down with your rockets. If you're successful, you'll get all three.
And seeing as you can do this mission at any point, I recommend completing "Arms Shortage" first, or, if you are aiming for 100% on this playthrough, collect all of the Hidden Packages as well.
3. Already been said, got nothing to add.
Alternatively, just follow the video guides made by DutchBudokaiFan on YouTube, and you'll get 100 Hidden Packages in no time. And you will complete the missions easier (although he doesn't know as much tricks as others do).