I don't remember all the details, but on at least one occasion, I've had the US/RO determine that it wasn't a competent RO for the application and it just forwarded the application to the IB. We were able to proceed through the IB. It may be that the client (Singaporean, if I'm remembering the correct case) didn't have a "natural" RO at the time in that Singapore didn't provide an RO. Maybe bleedingpen's experience is distinguishable in that UK has ROs other than the IB available.
With respect to (1), I've heard that there's some sort of patent agreement between the US and Canada, but I have no idea what's in it and whether it applies to PCT applications filed in the US/RO.
PCT Rule 83.1bis(a) says any person who has the right to practice before the national Office of, or acting for, a Contracting State of which the applicant or, if there are two or more applicants, any of the applicants is a resident or national shall be entitled to practice in respect of the international application before the International Bureau in its capacity as receiving Office under Rule 19.1(a)(iii). What, if any impact, does the "or acting for" have on this?
The way I read that is that it defines who can practice before the IB, not who can be represented by practitioners. Here's a hypothetical to explain how I read this. Imagine that the Philippines has no national patent office but that US practitioners can represent filipinos/filipinas in the USPTO due to historical ties between the Philippines and the US. Assume also that the Philippines is a PCT contracting state. In this scenario, the US national office (USPTO) would be "acting for" the contracting state of the Philippines -- US practitioners would therefore be authorized to practice before the IB even if the US was not a contracting state of the PCT.
Another way to put this is that anyone licensed to prosecute applications on behalf of residents or nationals of any contracting states, even if not directly in the national patent office of that contracting state, can practice before the IB.