You may remain and apply for citizenship when eligible.

No the wait times would be the same as any other sibling.

Yes this is possible. Best to work with an attorney to determine next best steps.

No you should wait until the I864 is complete before submitting I485.

You will apply based on the date of residence since on GC.

Yes you may do so and upgrade your petition once becoming a USC.

Selected as the best answer

No it is not required to inform USCIS of marital status change.

Once you marry you may initiate consular processing through the filing of form I130. Consult with an attorney to discuss further.

Too early to determine. Consult with an attorney to discuss your case.

Yes it should be okay to travel based on information provided.