I would classify my wife in the super responders category.
To attempt to answer the first question, it depends on a number of variables. To start with, assuming she has the same father for all her children, what is his genotype? R1R2 or R1r. It makes a difference.
On another note, FMH during the pregnancy will only be a factor if the baby is D positive and mom's titer needs a boost. If mom starts with a high enough titer FMH during the pregnancy is not required for sever HDN. My daughter was born with sever HDN and there was no known FMH during the pregnancy and all of the antibody studies suggested that she should not have been as affected as she was.
Bottom line, there is no cut and dried answer to your question and honestly, there never is in the wonderful world of blood banking. I suggest you get used to gray because black and white rarely if ever exists.