If I send new deposit on Saturday or Sunday - is it going to enhance my free margin immediately? Am I going to be safe on Monday, at market opening?

Deposits can be made over the weekend. As soon as deposit hits the account, balance and equity are updated, therefore enhancing free margin.

All payment processing is being done by the third-party payment processors (i.e., Bitcoin, Instacoins, Ethereum, etc.) and their functionality cannot be guaranteed.

Some payment methods are not instant and may take longer than usual especially during market opening when payment systems are just getting online.

Be sure to only place trades based on your current balance and not in anticipation of raising the said balance.