Good questions, but many of the statistics you seek, I'm afraid, you will likely find mostly meaningless. Communities each have a vast diversity in both number and potential for their ideas, so averaging them would be like asking how long the average road is. Not terribly helpful.
What percentage of proposals reach each phase?
There have been 3210 proposals submitted to date, but that bloated number is a gross exaggeration of what most would consider an earnest proposal. A lot of them were noise and unpursued ideas, so they were eventually abandoned, closed, and/or deleted. So Area 51 created 83 of our current sites (let's say another dozen or so closed after launch)… that makes the "success rate" about 3%. Not terribly meaningful.
What site holds the record?
The original Mathematica proposal was closed after 17 months, but the relaunch wins as the fastest proposal-to-launch time with a record-breaking 32 days. The next fastest times (last time I checked in 2010) were 36 days (Webapps), 36 days (Gaming), 38 days (Ubuntu), 41 days (Cooking), and 41 days (Gamedev) (source).