Comment on Basics on Options Shorting/Writing
1. Yep. Since you have written the option, you have already paid STT when writing. You don’t have to worry about paying higher STT, firstly because on buying side there is no STT and also since it expired worthless, there is no trade itself.
2. Yep
3. Margin blocked, I can’t tell you exactly. Use our SPAN calculator: https://zerodha.com/margin-calculator/SPAN/