1 helps me track how well my brand is working. The more 'cross mod' users I am getting the better a job I am doing of marketing my 'brand'.
3 helps me understand what mods I need to do a better job of 'marketing' in terms of getting the users to click install. Some people may never click install, and that is always there choice, but many just might need more reminding, or bugs fixed, or more attention in some way by me. You know this one Paul, it's why you put 'please click install' in the display as your mods install, and why you do a number of other things.

Not sure, as stated above, how it would be difficult since all the data is there and it's just a % that can be done with a simple counter of unique users in one of the postbit hooks that are obviously already heavily modified and very familiar to you anyway.

3 is also a stat I can calculate by hand if I wanted, so it's not hidden data or anything secret. It's just a pain to do by hand if you have a very long mod thread.

4, added by you, would also be helpful to track fall off rates (or growth rates) for mods, as well as potentially detect other patterns.