I've been meaning to play the stocks by saving up a few of the Lester missions just so I can multiply whomever's funds to grab the last few properties I, well, don't need but just want to have. I typically ignore the property requests as I go for my remaining collectibles anyway, so I can't imagine me getting much out of a golf club or theaters when I've only done either activity once or twice for the sake of 100% completion.
On the Social Club site, take note of colored sidebars next to images: those actions/missions are what are required for "100%" completion. You don't even have to do all the stunt jumps or collect the nuclear waste, thankfully, just a specific amount is required (25 for stunt jumps, 8 for knife flights, bronze medals for flight school/gun range, etc.). I know I'll end up doing everything eventually, but man, those things are best done over long stretches because I reloaded way too many times to do specific stunt jumps.
I used my turbo controller. The AI had some weird stretches where they could swim/bike faster than me during the swimming and biking portions of the race, but once everyone was on foot I got ahead of the two or three that I felt like were miles ahead of me during the bike part almost immediately, and stayed there and won the race.
Auto aim to remove any chance of me blaming the game for its failures, because it'll be all on me when I fail a mission. It certainly didn't help in Red Dead Redemption, that game is just ass to me.
If I choose to play with a mouse and keyboard on the PC version, I'd probably play with free aim. Given the platform, head shots just come naturally. If it could auto switch between two control schemes, then great, but I doubt that'll happen.