Q about US customs/immigration related to ME-B2 visa

[SauerkrautandTofuwurst]

We have a tricky question concerning our multi-entry B2 visa for the US. If you are a customs/immigration office you might know this and could help us.

If you are a foreigner and have gained some experience with this issue we would be happy to hear your story.

General advice or opinions like call this or that agency are not helpful (we did that already and it did not lead anywhere). We also visited the USCIS and that did not help either (by appointment only and the next one is in three weeks)

Facts:
We are currently in the US on a multi-entry B2 visa (The visa expires in 2024).
We are allowed to stay until beginning of Nov2014.
An extension of this stay appears possible but will cost 290USD which ruins once more our travel budget.
We were four weeks in the USA, visited for one week in Canada and when we came back to the USA, the border guy did NOT give us a new stamp! He claimed we should depart "meaningful" (whatever that really means) in order to get a new stamp - so we have hard evidence that it is not THAT easy as one might think ::

Questions:
If we go to Mexico for one week (time can vary if needed) and then come back to the USA. Will we get another stamp which allows us to stay for another 6 months (or whatever time we agree with the customs office)?

What is "meaningful"?

I really hope somebody can answer this mystery as I do not like to gamble...