I can't give you any legal advice, but I can tell you what happens in our situation...
DH's visitation lasts from 7 p.m. on Friday until 7 p.m. on Sunday. Although DH's ex is pretty flexible about picking SS up or dropping him off earlier or later than that, if SS has to be home at 7, that means he's in her custody at 7 (wherever they've decided to meet, if not at her home). DH doesn't get additional car time, and neither does she on the rare occasions that she drops SS off or picks him up.
As for step-parent visits, this isn't really an issue for us. I will pick SS up and spend time with him until DH gets home from work, or I'll take him for something to eat and take him home when DH has to go into work on the weekends, but I don't take SS for my own visits. I don't think SS would enjoy it much (he's 13 and doesn't like us cutting into his "friend time" with visits as it is) and I know DH's ex would be up in arms (she would prefer that SS and I have a more distant relationship - I think she feels threatened, though she has absolutely no reason to be). So it's not something I would be comfortable with, but I think for entirely different reasons than you're expressing.
It may be wise to meet with an attorney in your area to determine whether you should have a court order outlining your visitation rights, his chid support obligations, and any other issues that are lingering between you.