Module development and code questions

I was thinking about this today as I was adding reviews to my site. Eventually I am going to add a duplicate review. Is there a way for amazon items/reviews to check(by asin or something else) already posted reviews so as not to post duplicates? I could probably search myself but if it search when I previewed or submitted a review and alerted me to the duplicate, maybe with an option to overide and post anyway?!?

I'm creating a module that emails users once their account has been approved. To do this, I need some sort of way to mark each account to tell if it has been changed, and if an email has been sent. Could I use the users.data database field for this?

I'd like to try to do this without imposing a new database table. This is why I'd like to use something existing, like the users.data field.

If this is possible, is there a programmatic (perhaps through the drupal API) way to do this? ANy examples or suggestions would be greatly appreciated - thanks.

I'm converting a static web brochure to Drupal and one of the pages is a gallery (single page of thumbnails, which when clicked opens the full size image in a pop-up). I want to stick with Drupal 4.7 because it's such a leap in terms of usability from 4.6 but I can't get any of the gallery solutions up and running. Has anyone had any success with the 3 main gallery solutions?

Actually I've just done a fresh install and even the image module comes up with the following:

I have a simple one for experts. As soon as user logs in he would select a preferred category. That category will be used as long as he does not change it to other values. Is there a standard (technique/hook/API) way of managing such session variables in Drupal? I would want to use standard stuff if there are available.

I'm posting this here and not in "issues" since it doesn't seem like anyone else has this problem.

Event Module 4.6
Site-wide/Default time zone: "-5"
Event time zone input/display: "Use the sitewide time zone"
Configurable time zones: "Enabled"
My Time zone: "Default"
2 Test accounts: "Default"

I'm tinkering with my first module. Well, really tweaking the old sso module to suit my purposes and will likely eventually release it.

The existing module uses the _init() hook to run code every time a Drupal page gets loaded, regardless of the path. In general, newer modules use the _menu() hook to designate what to do depending on the path provided.

But I didn't see any documentation for the menu hook saying how to do two things:

1) Designate an action that should be taken regardless of the path.