Is there a limit to how long a packet could be used

yea, normally the anhydrous form of a hydrate readily reacts with any moisture in the environment, releasing lots of heat in the process.  

as the anhydrous absorbs the moisture in the environment to the point that it is fully hydrated, it will no longer take in moisture.

Yes there’s a limited time for the usage of packet

It depends on what kind of packet it is thermosetting or thermoplastic

air quality

poor drinking water

and heavy polution

can lead to poor health

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)