It certainly doesn't help that HTML elides multiple spaces into one, so no matter how many stray spaces you put in an LJ post, only one will come out of the other end unless you force it otherwise. Despite the prevalence of people in this poll responding that they put two spaces after a period, I strongly suspect none of them bother enforcing this rule in their LJ posts, and were only thinking of formal writing.

You were supposed to, and still are if it's a fixed width font. However, as typesetters would tell you, for variable width it should be one.

Your need for a qualification is largely my fault for having written the question unclearly. I only meant to refer to ellipses at the end of sentences in the question.

Good point. I should have been more careful in writing the question, either specifying I meant to refer to the mark commonly used at the end of sentences, or else done the question with check boxes.

I only use "dot" when verbally spelling out domain names, as, I suspect, do most people.

Use a proper typesetting language, e.g. LaTeX, which knows what a sentence is and can space appropriately automatically. There may be some way of making Word do it, but for the life of me I don't know how.