L1-B and i94 confusion

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Hi,

My L1-B visa is through and my assignment is from Oct 22 to Dec 21. My husband and daughter are applying L2 in a couple of days and are planning to come and stay with me from Dec 21 to Jan 5.  I have booked one way flight to US, waiting to book my return flight once my plan with family is confirmed. Below are my questions:

1. What will be the expiry on my i94 ? Will I get grace period post dec 21? Even if I get grace period according to i94, can I stay in US post dec 21 ?

2 . Should I book return ticket? Will there be any effect if I don book my return ticket at immigration? Can I tell at immigration that I will book return ticket after a month once my project is nearing completion?

3. How should I plan so that my family can come stay with me from dec 21 to jan 5? (They will be on L2 visa)

Please advice.

Regards