The SG readings are in this chart below. Yes, 1 bad cell can weaken a battery considerably and yea 1 weakened battery can shorten the useful life of the pack. Like a chain with a weak link
True true true readings can only be had with a temperature compensating hydrometer or by taking readings with the battery at 70* and washing out the hydrometer between cells .... But for all general purposes you can just run through the bunch looking for weak or low readings ...Good Luck