Which last 100 km are you doing? If from Sarria to Santiago, no fears, take two water bottles OR your Camelback; you won't need both. There are lots of places to refill either.
And no worries about mosquitoes. The only time I was bugged (pun intended!) was 10-15 km from Santarum, and that was by flies and that was in Portugal!
Depending on the size of your bladder--chihuahua-sized, like mine, or Newfoundland dog-sized, you will either find a bar or albergue to use the loo (I would buy a croissant or something if going into a bar--they are not public bathrooms and really are for the patrons, which you would be if you bought a drink or some food). Or you will look around to see if there is anyone coming and will slip in behind a tree if there isn't. Don't forget to pack out your TP. There are lots of towns between Sarria and Santiago, so you shouldn't have too much to worry about.
October, by the way, is a fantastic time to walk! It is still warm outside and the harvest is on. Have some roasted chestnuts for me...all we have here on PEI are horse chestnuts and they aren't too tasty!