Welcome to Steemit @onetree!

"Is there a limit to posts per day, and does that limit include comments as well as blogs." 
Approximately 4 posts per 24 hours won't cause diminishing returns to your payout. Comments are not affected by it.

"Is there a limit to the amount of posts you upvote? From how I understand it, it is better to upvote a post from a popular blogger earlier?" 
No limit, but your voting power goes down 0.5% per vote, you can see your personal stats here: steemd.com/@onetree.
Yes voting earlier is better, then it depends how good the post does. Currently to fight bots you get 50% curation rewards at 15mins of a post, and at 30mins you get 100% of curation rewards from your vote. Before that the % of your vote goes linear to the author.

"Can I still upvote a post after 12 hours"
Yes, votes that appear after the 12 hour payout, remain there for another 30 days and are rewarded afterwards. But remember everyone can only vote once on a post.

"Lastly (for now), once or twice I have upvoted a post and seen the earnings for that post go down, why is that?"
This I think only depends on the average Steem price the last 7 days. So it adopts to that. Votes shouldn't be able to put the value lower unless its a downvote.

"I read an article that Steemit is growing exponentially at the moment and I'm sure many the new users would love to have more information, as this can be a slightly more technical platform. "
This is true, and I would love to have some posts and guides about steemit with information to be stickied so they are easier for newer users to see. Maybe we will see some update in the future concerning this.

Hope this helped answer some of your questions. :)

great questions and good answers back fron@acidyo - thanks for both, helps me understand a bit more...