RE Marriage-based green card. How do I know if I need a US sponsor (Form I-864)?

Yes you need a sponsor. The person has to be a u s citizen or a LPR

Can I extend my R1 status?

An extension can be filed. You should make an infopass and go to uscis and ask if it wS a typographical error

Yes it will be revealed. Remember you were fingerprinted

You should contact and consult an immigration attorney

Does" in line for interview" is a sign for clearing my background check for N400?

It's good news just means your going to get scheduled soon

For form 130i petition does one need to attach all documents from options (below) or just one if birth cert isn't available?

It's usually one of them. But you should have an immigration attorney help you with this

If an SSBI investigation is going on, will that impact N400 application?

You are subject to both. You should consult with an immigration attorney

N400 Denial

I highly recommend that you speak to an immigration attorney immediately.

This is a question that you should discuss with your company attorney. And see what they tell you. We don't know what their strategy is

Letter for B1 Visa

Ok so you should let them no the exact length of stay for your trip. It's a good idea to have that letter with you