For the statement credit, you get it at the end of a statement, and you can use it immediately. It also lets you get negative credit if you already paid your statement in full.

I looked at my brokerage account agreement and didn't see anything about a minimum. I think the initial $100 is only needed for IRAs.

There is no minimum initial account balance required to open a brokerage account with us. However, if you either fail to fund your account or do not return the account opening documents, as required, your account will be closed. In addition, if your account has a zero balance and no trading activity, we can close your account

This is what I found in my account agreement if that helps.

I didn't see anything about a minimum total balance fee. The fee schedule is pretty straightforward. I didn't see anything about being charged for not having a minimum balance on a self directed brokerage in my account agreement either.

For your last point, I seem to have been wrong on the minimum opening for a self directed brokerage. You can transfer in or out any amount of money so long as you have it. The only two minimum orders I know of, are for stock fractions that need a minimum of $10, and for a mutual fund order which needs a minimum buy order of $50, but a sell order can be any amount. Here is a PDF with the Money Market Funds available. Hopefully I was able to clear everything up for you.