Margin trading is a pretty standard feature across all brokerages. Dangerous if used ignorantly, but greatly increases you returns if used effectively.
With sufficient cash flow, margins aren't that dangerous, even in a downturn. For example, if you have $24,000 and invest $500/mo into an account, getting a $6,000 margin loan in january that you pay off throughout the year isn't really that dangerous.
The problem with margin trading as a retail investor is that they interest rate on the loan is ridiculous, between 6-9%. Making it almost worthless.
It depends on the brokerage. There is Reg T. margin and then there is portfolio margin (the latter has significantly higher capital deposit requirements).
It also is not a straight 1:1 ratio. You generally can't go absolutely nuclear with leverage unless you have portfolio margin. Typically, brokerages will require collateral in the account equal to the risk (for defined risk strategies) and you will need a higher trading clearance on your account for unlimited risk.
Your buying power is a function of how long the trade will be held as well as the risk profile.
At least often, yes, but you will also need approval to even get that far. Etrade for instance has several levels of approval.
I note that Etrade at least implements it less as "margin approval" and more as "advanced mode"; to do any sort of options trades, even the ones that don't require margin themselves, requires you to activate the "margin feature" on your account.
Robinhood makes you accept a new agreement to be in Gold which sounds about the same as getting margin approval at a brokerage (agree to the risks and approval is automated). They don't have special powers to waive regulations.
