When people start making kefir they often worry that they screwed up. We understand; it can be an intimidating beast at first, with unfamiliar textures and smells, slimy grains, dried & cracked milk fat that's risen and separated. And you're supposed to drink this stuff? Yes!!
Evolution equipped us with a sense of smell that can guide us remarkably well. Ever smell spoiled meat, rotting vegetables or a rotten egg? You immediately know that it's bad, with no thought required. Think of kefir similarly—if it smells "okay," it probably is.
Properly fermented kefir should have bread-like yeast smell and taste, with some lemony acidic tartness too. If the batch smells wrong, throw it out and try again tomorrow. You can try adding more or less milk, or fermenting for more or less time.
Don't worry about killing the grains—it's pretty hard to do if you're making fresh batches every day. Just strain away the milk, optionally rinse the grains with distilled (or otherwise chlorine-free) water, add fresh milk, and try again. The resilient grains just need to get their proper yeast/bacteria balance back.
Kefir is a complicated microflora ecosystem, with dozens of good bacteria and yeasts living in symbiosis. If the grains are stressed somehow (too hot, too little milk, they just spent a few days in the mail, etc), that symbiotic balance can get out of whack. That means that the "wrong" bacteria are dominant, and so the smell is different. If you give it a few days of proper care, it will fall back in balance and start smelling "right."
Sometimes, but definitely not all of the time. In fact having some leftover fermented kefir in the jar can help kick-start the process (sourdough starters and sauerkraut are treated similarly: a little bit of the previous batch is added to the new batch to get things started). We wash the jar three or four times a week, and the grains once. But that's not a rule by any means...really we wash things when it's been a while, and time allows.