The answer to 1) is:
The installer doesn't use the same releases as you see on the download page, it has separately signed releases so they're not directly comparable. Sorry I don't know how you can predict when you might next get an update when you've used the installer.
The answer to 2) is:
Latest release for installer devices are here: http://wiki.cyanogenmod.org/w/CyanogenMod_Installer#Supported_Devices
For the galaxy nexus that is:
On that same wiki page it says:
How do I get updates?
Updates are done via Settings > About phone > System Updates. When a new snapshot build is >available, your will also receive a notification. Devices flashed via the installer will only get updates when a new stable snapshot build is available.
I suspect that might be out of date however.
The following seems to be the most recent up to date information:
Installer builds are those served via the CM Installer applications (Windows, Mac). These >builds are specially signed and named separately – there is no concept of an ‘M’ build for >this release channel – so saying ‘Installer skipped M#’ is an incorrect interpretation. >This channel only ever receives specially created Stable builds – they even come with their >own naming structure (eg XNPQ02R).
There is an additional blog post that rehashes some of the information on the m4 blog post above with a bit of extra clarification here: http://www.cyanogenmod.org/blog/cyanogenmod-11-0-m6-release