You can upvote 10 posts a day at 100%. Minnows don't have a vote slider so your vote is always at 100% since the hard fork. I'm not sure why they didn't fix that yet.

The receiver will always get credit but it's better for your rewards if you wait to vote at 30 minutes .

Your steem power is not reduced when you vote. That is real money that will always be yours unless you actually transfer it. The amount of money you give in relation to your steem power goes down after your 10 votes.

You can upvote old posts, but after 7 days the author does not receive any extra monetary reward.

I'm going to stop there because if I answer all your questions I'll have written a complete post. There is a FAQ you can check out that might answer some more.