McLaren's contract with Mercedes is optioned till 2015.
Do you really think Honda wouldn't provide their engineers direct access to look and study engines McLaren has purchased from Mercedes?
I think there would be something specifically in the contract between McLaren and Mercedes (indeed, any engine supplier and customer) that disallowed this. It would be up to McLaren to decide whether or not they wanted to take the risk...
The things that concern me are the fact that the previous Honda engines weren't the best, and the company's long-term commitment to the sport if they do come back.
The Honda V8's lacked power cause (supposedly) they observed the engine freeze much more strictly than some other manufacturers did. Toyota suffered the same fate. I don't think the Honda engines were unreliable or anything in 2007/8 either, although i can't really remember!
It wasn't just power that they lacked, but consistency. The big comments that came from both Button and Barrichello when the Brawn car was fitted with the Mercedes engine was how much more driveable and predictable it was. Then there's also the supposed drops in power after each use, which were considerably high for both Honda and Toyota, although that could be down to the reason you mentioned.
I think the rumours of Honda staff already at Woking is just the German press being naughty. After all they see Japanese technicians behind every single cabinet in every factory
What could they do before 2014 when Merc deliver the new engines anyways?
Work with McLaren to determine the kind of dimensions that would be required for the 2014-onward spec? They could do a lot of work based on the WIP car in terms of how to package the engine, even if they're not able to test anything physically so far.