On expiry day, for short option position, the margin requirement will increase to 40% of the contract value or SPAN + Exposure margin (whichever is higher). Though you’ll continue to get the margin benefit.
For your physical settlement obligation to be netted-off, both legs should expire ITM. If one expires ITM and other OTM, it’ll be physically settled and depending on Option type, you’ll have to give or take delivery of underlying shares.
Yes, taki Long Option position will give you margin benefit. However, for settlement, again it depends on where the legs expire, as explained in point #2.