Back when you were posting in the other thread I did some Googling to try and find your bike. I never did. What year is it, do you know? Most of the Roubaix's I see online are newer and TBH yours looks way nicer than any of the ones that come up in images. If you bought it from a dealer they should be amenable to getting you properly fitted. Even if you bought it private sale, a Specialized dealer should continue to honor the implied warranty on the original sale, especially for a low cost operation like a sales fit. Quick release seatposts have a threaded adjustment on the other side of the cam lever and if yours is slipping you need to tighten that adjustment just a little so the cam lever holds better. In my city keeping the OEM q/r seatpost collar means the loss of a sometimes pricey saddle very quickly. You only do that once, especially when its a tandem and that seatpost and saddle were attached to a very spendy Controltech Stoker stem and Stoker bullhorns.