#1 just change name at Social Security. That's where the IRS matches. So use your ssn & Married name now.
#2 you can leave your W4 as Single. You can put down anything on the W4. It doesn't have to match your actual status or tax return. I would leave it at Single to have more taken out now that you have 2 incomes being married.
#3-5 no problems see #2
You will file your tax return as Married filing Joint
or Married filing Separate. It is usually better to file Joint. Joint has the lowest tax rates and the highest Standard Deduction. And if you are in a Community Property state MFS gets tricky to figure out. There are many disadvantages to filing MFS and you can't claim several credits.
See …….
https://ttlc.intuit.com/questions/1894449-married-filing-jointly-vs-married-filing-separately
