1. The cross out is achieved by the proper html tags. I believe the one in question is <STRIKE> Like this Perlmonks allows certain HTML tags in posts (indeed requires a minimum knowledge, which you seem to have :) )
2. As an unwritten custom, Monks don't like to delete info. A post you made may already have comments from others. Rather than leave bad info out, it's customary to correct any bad info, and leave an indication that you did so. Sometimes through strikethrough, sometimes just with an brief note labeled "Update" at the bottom.
3. Note that you can (in general, I believe each section has it's own rules), edit a comment in a thread, but not the node that begins one. Thus, you can edit your question that I'm replying to.
4. Reputation comes in two parts: A node gets voted up or down by other users on Perlmonks. (When to vote up or down is the subject of much debate, but in generic terms up is good and down is bad. The sum of plus and minues is the nodes repuation. The other half is a user's reputation. As you write nodes that gain or lose reputation, some of that reputation is associated with you. (Generally this is referred to as Experience). As you gain experience, you will increase in level, until you achieve Sainthood. There are a few Level Powers associated with making levels, but in general it's just an indication of how much a given monk supports perlmonks. (At your next level, which doesn't take long, you will gain the ability to vote on nodes, for example)

Hope that helps!

Update: Here are some links that might be useful:

What is reputation?
Voting/Experience System
Number of Monks by Level