Is there any difference between the words lowest and lowermost? When should I use either of them? Possibly lowermost should never be used?
Lowermost makes more sense when used to reference actual physical locations, "the lowermost floor." Lowest can be used more generally to compare, " e.g. "the lowest ranking officer."
OALD has no entry for lowermost. Actually this means you don't need the word. Collins and TFD (The Free Dictionary) have an entry for lowermost meaning lowest. So lowermost is just a variant for lowest, and the question is whether such a variant is necessary at all.
Ngrams shows that the frequency of lowermost is extremely low. Link