L-1 visa renewal for the short time left (4 months)

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I started working on L-1B visa in the USA from April 2015; I renewed it once in Dec 2017; The visa is now valid until April 2020;  But the I-129S and I-94 are valid only until Dec 2019 (for both me and my wife). Myself and my wife are planning for a baby before leaving the USA. (we will come back to USA after working 1 year in home country with a new L-1 visa or H-1B); I have a spouse on L-2B and a US born son; we do not want to be in a position that requires travel during later stages of pregnancy; I know that L-1B visa is valid for a max period of 5 years (so until April 2020 for myself); 

1) If I file my extension 60 days before expiry of I-129 S in Dec 2019, will the USCIS extend my stay until April 2020?

2) After filing extension and if I get approved can I stay until April 2020 without going for stamping?

3) If my visa gets approved what is the minimum period that I can stay in the US before going for stamping?

4) Is there any other way to extend my stay such as changing to H-1B from L-1 etc?

Thank you very much in advance

 

 

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Is this a blanket L1?

Does the company have no say in your decisions ?