In times of utter desperation, it is possible to rehydrate oneself by consuming one’s own waste fluids? I read somewhere that urine is sterile, so can one drink urine (as a last resort) as a source of water?
Not without first distilling it.
Urine is a liquid containing stuff your body is trying to get rid of; salts, acids and the like - when you are dehydrated, the concentration is likely to be significantly higher and so, by drinking it, you add the waste products back into your system and increase your need for water to flush them out.
So you need to make a solar still like Kevin Costner has in Waterworld 
Effectively it is the same as drinking sea water. It doesn’t solve your dehydration problem, just makes it worse.
Yes, but in many cases there are simpler methods of getting fresh water; if you have a clean plastic bag, you can just tie it around the leafy end of a tree branch and condense/catch the water evaporating from it - no need to muck about with urine.