m4816k,
Here's my take on a couple of your questions--
"tooth test" best done pearl to pearl--on the same strand (Betty Sue King style.) It is very subtle grittiness felt through the fingers, and if you have a faux strand of pearls to compare the feel with, usually they are very smooth, it helps. It is just one factor to consider. Usually there are more definitive visual clues as to whether a strand is faux or real.
Dyes--I feel most are pretty good, as factories have been doing this for quite a while now. If you are buying very inexpensive pearls, however, and when you look closely there appears to be areas where there is more dye, often the less perfect pearls are dyed, give them a gentle bath in castile soap to check for color loss. Remember that the string is often dyed also and will release color.
Wearability and durability of pearls often has a lot to do with the wearer, how they understand basic pearl care, and how the pearl is mounted and what kind of setting.
I have never seen freshwater mabes-----------
Just my opinion------