Need advise on travel. L1A expiring, EB1C I-140 pending

[workhorse]

Current status L1A, exp 9/5/08 (family on L2)
Pending: I-140 EB1C, RD 3/5/07, still no news from Nebraska

Fact: My family members MUST travel to Europe this summer.
Fact: My extension I-129 for a renewed L1A must be submitted to USCIS no later than 6 weeks before 9/5/08.
Fact: I have L1A visa stamp and family have L2 visa stamps with expiration date 9/5/08 in our passports

Question:
How late is the latest my family members can enter US without encountering trouble at the border?
They need to be present in the US to submit the I-539 (extension of stay), but can it be submitted AFTER my I-129 or MUST they be submitted at the same time as an absolute rule?

Please advise.