My experience with RRL is that women prone to miscarriage should not use it in the first trimester and that women with a history of preterm labor should use it cautiously. You are right that it is a toner and not a stimulant. My general description of RRL is that it "reminds the uterus of what it already knows how to do". It will not "cause" contractions in the way that black and blue cohosh will actively cause contractions. However, if you have an irritable uterus or are prone to pre-term labor, sometime RRL can "remind" your uterus a bit too strongly of what it is supposed to be doing.
If I were your midwife, which of course I am not so I offer this only as a piece of information and not a "you should do this" kind of statement, I would want to look more specifically into the circumstances around your preterm birth. Sometimes women go into preterm labor because they have an irritable uterus or "incompetent cervix" (I HATE that term btw!! no cervix is "incompentent" in my book). Sometimes there are environmental factors like stress, poor diet, illness, etc. And sometimes baby wants or needs out for his or her own reason. How many children do you have? When were the other ones born? I think she is wise to be cautious about RRL. Personally, I would probably avoid drinking the RRL leaf tea. Maybe switch to some nettles instead which will strengthen and tone and keep you super nourished. I would, personally, be less concerned about the prenatals. I think that a small amount of RRL is okay and potentially even highly beneficial to keeping your uterus strong and healthy throughout the pregnancy. But be self aware and if you notice a lot of braxton hicks or crampiness, then I would highly recommend switching prenatals. Of course, I think it's most important that you and your midwife find something that feels comfortable to both of you. If you really want to continue taking the prenatals with RRL then talk to your midwife about it, discuss your thoughts/concerns and see what she thinks. You may also want to talk to your midwife about starting RRL at 36 weeks. That way there is no concern about pre-term labor but you get to boost your system with some awesomely nourishing herbal love as you get closer to delivery.
Best of luck mama....sending some happy uterus vibes your way :)