Looks like I'll need people to help grind me up in MP after all. I spent about 7 hours trying to do missions in an invite-only session, but I'm getting torn to shreds in Gerald's missions and I'm having extreme difficulty catching the cars in Simeon's because my driving skill is so low.
When I say "torn to shreds" I literally mean that one or two bullets eats through my body armor and another one or two kills me outright. I've managed to do one mission successfully but not the rest. If there's a car involved, it's still the same - they pull off headshots and I'm dead almost as soon as I see the red blips on radar.
In one of Simeon's missions, I spent 2.5 hours chasing a single car around the island. Five laps, not counting the back-and-forth I sometimes had to do. At the end, the target car crashed and burned, I got $200, 200 points, but had to spend $325 getting my car back after I slipped on sand and tumbled off the cliff into the ocean (it was the windy cliff near the lighthouse).
So after these 7 hours, I've gained 1.5 levels and made $3k. This totally isn't worth it...