With CYA around 45 your equipment should be totally safe with FC levels up to the mid-twenties. The only possible problem would be if the Nature2 unit has gotten the copper level very high, which is quite rare, but could cause some copper staining at extreme FC levels. If you stick with normal shock levels, around 16 to 18, you should avoid any problems with copper.
Your very low FC levels could have something to do with a broken chlorinator and/or the heat. However, once the FC level gets to zero for any reason, you will start to get algae and then need to shock the pool to be sure it doesn't ever really get going. Once any possible algae is cleaned up you can check to make sure the chlorinator is working, and/or switch to a different source of chlorine.
I am concerned that your CYA level might actually be higher than 45, as several others have mentioned, which could cause some additional complications.