Only if she has Earned Income greater than $6,200, or Unearned Income (interest, dividends, capital gains), or she has self-employment income greater than $600. However you may want to file to get a refund for tax withheld.
$17,500 if you are under age 50, $23,000 if you are over 50 year of age.
401(k) if you have the option at work. A traditional or Roth IRA. If you have your own business you can have a SEP, a Uni(k) or even a Defined Benefit Pension Plan.
No. If you are married you can either file Married Filing Jointly or Married Filing Separately. Your filing status is determined by your marital status on December 31 of the tax year.