Aye. When it asks for "Start Position" then scroll sideways and one of the options should be "User Defined" (the others are "Start", "Last Viewed" and "End") , at that point key in the time in minutes that you want to go to.
I got the 100%. It never felt like a grind apart from collecting the nuclear waste - it took me almost two hours to circle the island in that poxy submarine.
I played it for about an hour this morning and I didn't have any issues. I spent most of that time killing AI with the broken bottle, it was a lot of fun.
Finally got 100% on the single player. Not the kiddy-on "100%" that gets the achievement but the full monty.
I suppose I should try the online stuff now.
I was concerned that that would be the case. Red Dead Redemption's passive mode suffered because folk couldn't kill you but they could shoot your horse. It was a bit of a nuisance.