Experts invited to comment - 2 H1Bs and stamping

[rashiars]

Here's my situation:

First I797 (same company same end client - EVC): valid until 11 oct, 2011. Passport not stamped with this visa

Second i797: valid from 1 oct 2011 to sept 30, 2014

I am planning to go to Canada for stamping with the latest 797 on Sept 12 and come back on Sept 15 questions:

1. Can I do this? I.e have my passport stamped prior to the actual start date of my new I797?

2. What date will they use to stamp my passport? Sept 12 or the start date on latest 797 i.e. OCT 1. If sept 12 then should I carry all my old LCA n 129 for the visa interview etc?

3. This is prime question that tops my concern:

If I get stamped for oct 1 then how is coming back to the US work? My passport will have a start date of oct 1 can I enter on my first i797's i 94 which is still valid until Oct 11?

My head is hurting real bad, I may be confusing myself and the solution could be real simple so, I request the gurus to get me some clarity!

As always appreciate it!

Best,

RG