Firstly, respect for your initiative in seeking an alternative route. In the end I suspect if there isn't a real regulatory flaw in the plan some jobsworth will find an excuse to scupper it anyway, but best of luck. FWIW I have a couple of observations:
2) Go to the US, do a full FAA IR course and pass the tests (with all the visa red-tape involved etc)
If you are a UK citizen you don't need a visa for such a short course. Last year I travelled out on an ESTA and completed TSA checks after arriving at my flt school. It takes 3 days or so (you can do the checks before going if you wish but my school advised against this in case I cocked it up).
3) Possibility to train for the FAA IR in a “condensed” and focused manner (i.e. many structured FAA IR courses with 2-3 week duration are offered) fits my lifestyle better
Well, focused is definitely good but imo that's a bit tight, so I would suggest you schedule a good buffer in case of delay, and a flexible return ticket is worth the extra expense. You don't need to be stressing about running out of time. If you get it all wrapped up as quickly as advertised then bonus.