dear Bettina, there are so many things to consider  in these cases but here
are a couple of thoughts.
1. large breasted women (or ones with problems with size, this one sounds
like a good example) absolutely must have a decently fitting bra,  you're
right about this aspect. just don't ask me where to find one.
2. Adhesions could cause this continuing pain, as could muscle/back pain from
strained or tense nursing position that is expressed as breast/nipple pain.
3. i am puzzled about your idea to try to inhibit supply, this always worries
me. does she have symptoms of overative letdown, oversupply ? these can be
managed in a more mechanical fashion.
Several statements made by the mother remind me of another case in which the
mother stopped breastfeeding but pumped her baby's milk supply for almost a
year. Severe psychological problems became more and more obvious as the
reason for  her similar complaints about nipple/breast pain that could not
otherwise be explained. The complaints about fullness even after nursing
could relate more to difficulties with the change in the mothers body and her
feelings about it than to a specific physical problem (i.e., oversupply).
Another case i dealt with was so similar to your mother, except that this
mother also kept saying (after difficulty getting baby on, pain, etc.), "she
doesn't like me." Needless to say, i assume she weaned early.  Good luck!
Carol Brussel, LLLL, DA