Change or Not change W4

#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

The W-4 you fill out for your employer is only for your employer so the employer will withhold the amount of tax you want withheld.  The IRS does not get your W-4.  You can change it or leave it as is.  You might want to look at/use the IRS W-4 calculator to decide if you want to change anything.    There are no "penalties" for the way you complete your W-4.  You just want to have enough withheld so that you do not owe tax at tax return time.

https://www.irs.gov/individuals/irs-withholding-calculator

When you file your tax return, you will want to use the last name that matches your Social Security card.

As for filing married fling separately or married filing jointly,  filing joint is almost always better, especially if you have children.  You lose a number of child-related credits if you file separate returns.

Married Filing Jointly is usually better, even if one spouse had little or no income. When you file a joint return, you and your spouse will get the married filing jointly standard deduction of $24,000 (+$1300 for each spouse 65 or older)  You are eligible for more credits including education credits, earned income credit, child and dependent care credit, and a larger income limit to receive the child tax credit.

If you choose to file married filing separately, both spouses have to file the same way—either you both itemize or you both use standard deduction. Your tax rate will be higher than on a joint return. Some of the special rules for filing separately include: you cannot get earned income credit, education credits, adoption credits, or deductions for student loan interest. A higher percent of your Social Security benefits may be taxable. Your limit for SALT (state and local taxes and sales tax) will be only $5000 per spouse. In many cases you will not be able to take the child and dependent care credit. The amount you can contribute to a retirement account will be affected. If you live in a community property state, you will be required to provide additional information regarding your spouse’s income. ( Community property states:  AZ, CA, ID, LA, NV, NM, TX, WA, WI) If  you are using online TurboTax to prepare your returns, you will need to prepare two separate returns and pay twice.

https://ttlc.intuit.com/questions/1894449-married-filing-jointly-vs-married-filing-separately

https://ttlc.intuit.com/questions/1901162-married-filing-separately-in-community-property-states

https://ttlc.intuit.com/questions/1894449-is-it-better-for-a-married-couple-to-file-jointly-or-separately