Yes, when you sell shares that are already credited to your demat account, you will be 80% of the sale proceeds on the same day, these can be used towards any trades.
Yes, if 80% of the sale proceeds are the only funds in your account, to buy back the all sold quantity you will need to add additional funds as 20% are blocked and available only on T+1 day.
You can pre-authorize sell transactions from 8 AM onwards. More details here.
The profit you will be making on the long option position is unrealized and it isn’t added to your account balance until you exit the position. So if the futures position is on loss, you will have to bring in funds to settle this, if you don’t have funds in your account.