I think 24 months(as opposed to 12 months) drop box eligibility will end on March 31st 2021 as far as I remember. So if you go in May 2021, then you might be ineligible unless they extend the 24 month rule.
You can simply go for H4 stamping directly when you are in India, not required to apply for H4 Extension. And after coming back to US, you can apply for H4 EAD.
Yes you can fill DS-160 before marriage with marital status as Married and also can take an appointment before you actually get married for the date past marriage date.