In addition to concern about antibiotic immunity, can low dose long term cefalexin cause other problems and, if so, what would help?

Yes.: YOU don’t get immune to antibiotics, the bacteria do (and then become unresponsive to further antibiotic therapy). Low dose, long term cephalexin is an “ideal” situation to encourage bacteria to become resistant. Need to fully discuss with your doctor (or an Infectious Diseases doctor) why you need “low dose, long term” cephalexin - very few such situations