Hi Chris,
1. Any host that supports mojoportal should be fine but how many sites you can host and performance will depend on the robustness of the hosting package and the aggregate traffic of all the sites. I don't really have a report card for different hosts. Have heard some complaints about WebHost4ASP recently but used them in the past and they were decent. For shared hosting I currently use DiscountASP or CrystalTech, for dedicated servers I use 1and1.com. I'm in the 1and1 affiliate program so if you go with a dedicated server from them please click this link and I can get a little commission. I'm not an affiliate with any others, I just list hosts that I'm aware of that do host mojoportal.
2. It can be done, its not trvial but its not extremely difficult. You would backup the site file system and database and restore in a new location then delete unwanted sites from the db using Site Settings in the root admin site, then delete unwanted /Sites/[SiteID] folders. Probably would be best toi query around in the db and make sure all unwanted site data from other sites is gone. Once upon a time joeaudette.com was running from the same installation as mojoportal.com but I separated them a good while back with this approach.
3. the IIS logs would include all sites traffic because its really all one site from IIS point of view. But google analytics and woopra work just fine because they don't rely on IIS logs they track to their own data from javascript. ASP.NET doesn't have any logging for web requests, its IIS that does the logging. The mojoportal log is also per installation so events/errors of all sites appear in the same mojoportal log. Only the root admin site has visibility to the mojoportal log via the UI.
I generally don't host different customer sites in the same installation (though some people do). I host multiple sites for the same customer that way sometimes. There is no good way currently to backup or restore a specific site from a multi site installation. If you restore the db its restores the whole db.
Hope it helps,
Joe