Eric,
You're right on track. Tvix doesn't have an ATE (automatic termination event) like other ETNs, but can still be delisted at ANY time. A few Barclays ETPs hit their ATE this year and were delisted, but I don't remember the specific symbols at the moment as I try to stay with safer ETFs. Could happen to TVIX essentially any day, and is one of the reasons holding leveraged volatility products overnight "can" be risky.
Im a HUGE fan of SVXY. Way better than XIV. Follows VIX closer, Is an ETF and surprisingly mirrors UVXY in comparison to other "paired" products. I actually use UVXY options to partially hedge my SVXY positions.
Now, as for why UVXY doubled. First thing you need to remember is that these products are NOT based on the spot price of the VIX, but rather the VIX futures. So basically the bad things about UVXY during contango (negative roll yield, etc etc) become the good things in backwardation. This acts like a slingshot, where the longer and steeper the backwardation the more multiplied the increase in UVXY. So say this fiscal cliff deal fails and we get the steep rise in VIX for all of January. Backwardation could continue into February and UVXY could easily reach $100 were that to happen. Likely? No. Possible? Definitely. The interesting about these volatility products though is mean reversion of the VIX. Were that scenario to take place, UVXY would be back to <$20 by March, for the same reason why it went up. This is even more of an effect since it is a leveraged product.
Does this make a little more sense now? This is why volatility products are so enticing. Invest wisely and they can be the closest thing to "long term risk free" on the market. Essentially the only thing that can destroy these funds is a black swan event, in which case we're all screwed. I do recommend monitoring positions daily at a minimum, and ensuring to retain enough available capital to adjust positions as necessary.
Oh, and stay away from TVIX. It is backed by Credit Suisse. The block on new share creation was a failed attempt to save the fund. Everyone was shorting, no one was buying. That's how hedge funds fail too... If you want to get in on the <$1 action and that's why it is enticing to you, wait for backwardation. Cool thing about these products is that once backwardation is truly entered (only hit about 2 days in November), it can take days to weeks to months for the funds tracking the VIX to reach their highs or lows (if inverse). So don't establish a position until solidly in backwardation, and you can still make potentially 100s% return with far less risk.
