1. For KPCB to exit, they need to find an investor that values the company more than they do. To find an investor who values an unprofitable picture sharing company at over 10B would be a difficult task. It will probably be possible to find retail investors to provide the exit strategy, but the big tech companies of the world (Google, Facebook, etc.) do not have incentives for buying out unprofitable poor performing companies with no future. They have an agenda, and their CEO's are looking for strong future revenues because they themselves are heavily vested in the companies which they work for. These are savvy people with a team dedicated to valuing M&A transactions. There is no good reason for a large tech company to acquire a snapchat unless it thinks it will contribute to the bottom line, the brand, the network, the IP, or the scale of the existing business.
2. There's this incorrect notion that VC firms are somehow not exposed to the risks of the market. Times are good right now, and if you're a VC firm, you should be making a profit right now. A typical pump and dump occurs because of timing effects, insider knowledge, and mass marketing. While you could argue that an IPO is a "mass marketing", I think that external financial institutions (hedge funds) are very much looking out for shorting opportunities. An overpriced IPO is a great opportunity for a hedge fund. Also, It's worth mentioning that the VC firms which will profit so nicely this year, are likely to lose 50-80% during a downturn. They are very much exposed.
3. If KPCB really wanted to profit, why only invest 20MM? This won't make them much money. The real reason they made this investment is because they're skeptical of snapchat, but still want the talking point if there is an upside. It will be a great story for investors, pretty much regardless of how snapchat exits.