If the cervix is scarred such that it doesn't open on its own, or is difficult to manually open, then it is going to be an issue. If the cervix is not opening appropriately then that could be the reason the mare is retaining fluid. First things to do would be a culture/sensitivity and a biopsy. You want to know if you are dealing with an infection and also what kind of shape the uterus is in. If you have an infection, it will need to be addressed, and if the uterine biopsy has a terrible score then perhaps the cervix is a moot point. As far as dealing with getting the cervix to open and dealing with fluid retention there have been some good discussions on this forum in those regards. You'll want to utilize oxytocin and probably some type of medication (such as misoprostol) to assist the cervix in opening. However, in some cases scar tissue is going to preclude the cervix from being able to open normally, even with medical assistance.
Any specific questions I should be addressing with the specialist?
If the birth of the last baby tore the cervix, what can we expect during the delivery if we are able to get her in foal again and she's able to keep foal to term (which has never been a problem in the past)?
Inflammation and infection can also lead to cervical scarring, I understand. Is there anyway to tell how the cervix was scarred for certain?
We're not willing to do anything risky, that's for sure.
You could ask your vet to do a speculum exam to ascertain the extent of the damage to the cervix. It doesn't truly matter what caused the scarring, as the outcome is still the same. But it is why a culture/cytology is important b/c if there is an infectious problem then that is how it will be diagnosed. We have worked with a lot of mares with either a torn/scarred cervix or a cervix that just doesn't open and never had a problem with them foaling before. That would be a question for your vet in regards to your particular situation.