The big question is whether work on the USA side ECLSS systems has
completely stopped with all the gear stowed in some wharehouse, or
whether there is stull R&D and testing work being done. In other
words: is NASA taking advantage of this very long delay to ensure that
the systems that will eventually go up are extremely reliable ?

Well, I don't know what work is currently going on, but the key word
here is "eventually". Unless something has changed, the US ECLSS
equipment is all planned for the Hab module (or Node 3, or whatever it
is called this week). Which isn't even on the assembly sequence (I
was looking at http://spaceflight.nasa.gov/shuttle/future/index.html ).

I won't say it is impossible to do work now to improve equipment which
won't be used for 5-10 years, but I will say that having someone who
is using (or soon will use) the stuff you are building tends to
improve the quality of the engineering. At least, that's been true on
projects I've worked on.