Filing Extension While Waiting for My Wife's SSN

[da182]

I (a non-resident alien) got married last year and my wife has applied for her SSN but hasn't received it yet. She hasn't earned any income in US or abroad in the past year. 

I am planning to file for an extension and then wait for her SSN to arrive. We will be filing "Married Filing Jointly" (MFJ)
I am a bit torn about how to do the extension though:

1. Could I file an extension just for me or "Married Filing Separately" (MFS)? This will be convenient since this can be e-filed. And it looks like it's legal to change to MFJ when we actually file our returns post the extension.
              a) Since my wife hasn't earned any income, I am assuming she doesn't need an extension?
              b) Also, I am only planning to pay the tax amount that are due when filing as MFJ. Is this OK although I                        am filing the extension as "MFS"?
2. Should I instead file by mail using a Form 4868? This is definitely the safer option although I will need to file by mail.
Thanks a lot for any responses!

[LeonardS]

When you file for an extension you only have to provide your social security number, the form does not ask for your filing status. 

You do not file a separate extension for your wife as you will be filing jointly.

Yes, you may pay the tax amount owed based on your filing status.

E-Filing Form 4868 is just as safe as mailing the form.