Pardon me if I answer first to question 2, then to 3 and last to 1.
Question 2.
I would not worry about conflicts. If you assign to a subfolder a territory as target, Google is quite good in following the indication you give. Therefore, if the main domain .com is generally focused to co.uk and the /us sub-folder to the USA, Google will not have problem in targeting correctly the two. Just a suggestion: eventually specify in the header that the language used in .com us en-uk and in the /us is en-us... not that Google is really paying a strong attention to that signals, but I would use any possible additional suggestion of the target.
Question 3
You are right... but, if there aren't other reasons you don't want to show to an USA users the Uk version of the site, why don't you think to put the links to the other language/territory versions of the site in a "About us" page? In that page, editorially you could list the markets the site is active, and those link would be direct.
I say this also for an usability reason. I don't know how the site is going to be developed, but maybe it should be interesting for an usa user to read the british blog of the website...
Question 1
I agree with the USA guys: subfolder is better, because that way it starts his online life with a strong authority domain, so that any links the sub-folder will obtain will have a bigger power than if obtained being an sub-domain.
About your Panda fear.. .you are right to fear it, but the reaction - IMO - is not the correct one. If you fear Panda you don't have to react in a defensive way (going sub-domain), but in a pro-active and positive one: creating worthwhile and useful content, avoiding thin one, paying great attention to any duplicate issue and checking any other pandarank factor.
Ciao