The problem with repeated "I am... " sentences isn't that you're not varying your sentence structure, it's that you're not communicating the underlying meaning with your reader.
Consider the standard Essay format:
By itself, each "I am..." statement is an introduction to a fact. If you don't elaborate on that statement, the ...
If the blog does not have Terms of Service which require commenters to surrender their rights to the website owner, it doesn't matter who owns the website; it's still the commenter's intellectual property. They are choosing to have it distributed via the medium of your website, and they can revoke that permission.
Without a ToS clause, you do not have a ...
Kate, you can assume that a certain amount of reproduction from your web site to your book is fair use. They did, after all, post them in a comment on a public place owned by you. If their writing is substantial (i.e., pages of material), however, you may want to seek permission.
Another way around this is to paraphrase their comments rather than quoting/...