The placenta will have to be anterior AND low-placed--on or very near scar--to cause the kind of postpartum hemorrhage and/or placenta detachment problems mentioned. If it can be determined that placenta is on or near your scar, then you will want to know whether your midwife has ever done a manual removal of a placenta (which usually, but not always can be done for a sticking placenta)--along with having ways to help control a bleed. The excess bleeding is due to a placenta that will not fully release from uterine wall--so, bleeding may be difficult to control if placenta is adhered to scar. Just depends how much it is adhered--this can vary.
As for posterior presentation, well--there are several theories running around, on why some babies get themselves into a posterior position. They all make sense to me in certain ways--but we have to remember that none of it is proven, it's ONLY theory. I personally believe that posterior presentation, like all else that comes with life in a body, is the result of combined forces of the body and mind. I would no more want a woman believing that 'anterior placenta means posterior baby', than I want her believing 'once a csec, always a csec' or 'only hospitals can safely manage birth'. We need to remember that most babies present anterior, NOT posterior--whatever the woman's pelvic shape or size, for instance, or regardless of whether she has an anterior placenta or sits often in a recliner (recliner use is another 'culprit' named in posterior births).
Information is definitely a good thing! Being proactive is a very very good thing. But worrying is not a good thing--and visualizing/fearing the worst is also not a good thing. Maintaining a relaxed, trusting emotional and mental state,talking to yourself and your baby about a happy normal birth are the best things anyone can do for self and baby on every level (like, visualizing yourself handling contractions well, and visualizing your baby finding the best possible position for it's descent).
I do see that some pp's on this thread have said that their anterior placenta did not cause problems, and that is wonderful. It's so good to have this information from those who've btdt! So, gather information--from spinningbabies.com, from methods of discovering placental placement, and be proactive in your preparations...be ready for the possibility of adhered placenta by knowing what you and your mw may want to do about that if it should happen. Be ready to manage posterior labor, should that occur for you--and know that a posterior baby does NOT necessarily mean 'long labor' or 'excruciating labor'. Just try not to fear any of it--and remember that theories are just theories, and that your relaxed, open, trusting state is the best thing you can do for yourself and your baby as you approach birth.