Simply file Supplement A as members of the same household.

No immigration law consequences for failure to complete such questionnaire.

Is it necessary to file taxes jointly when applying for US citizenship?

Why not absolutely necessary, it is nevertheless expected and the norm with USCIS.

Recently arrived in the US. Is my wife eligible for economic stimulus check ?

Wife does not qualify for any stimulus check yet.

Yes, claiming "no income" on your tax return could be used as proof that you did not maintain valid O-1 status.

Process for F1 student from India right now studying Bachelors in the USA to transfer to Canadian university

Citizens of India need to apply for a visa to enter Canada.

Make sure to have a lawyer properly prepare you (and review the documentation for the I-751), as well as accompany you to the interview.

Highly unlikely that FL DMV will agree to renew/extend DL without valid EAD.

L1A - Clock reset query:

Have to first prove remained outside the US for at least one whole year.

My family struck in India those who are on Green card?

Strive to return before having remained abroad for one year. Flights should resume soon.