You can sit FAA writtens at certain schools that are LaserGrade (or rather, PSI now) or CATS. It's usually at certain premium outside USA. Up until about year ago, it was less, now 150 bucks in the US.
I can't tell you'd fulfill all requirements for FAA ATP regarding FAA crosscountry.
You do look like you'd only need the minimum training/flight review after validation for PPL(H). It's usually 3hrs preceding 2 months before checkride/flight test.
I don't see a point trying to do FAA ATP if you only need CPL
Those have to be with FAA CFI, other training will be credited, as long as the requirements as to the kind of time is met.
Also, initial FAA licence and instrument, you'll technically have to pay 130 bucks each, for TSA 'clearance'.
Doing flight tests in UK or France is possible with freelance DPE, but the fees are hefty and you'd have to be sure you got all paperwork correct.
Plenty info if you search, look at e-cfr.gpoaccess.gov as linked already. part 61.129 has the flight experience requirements for CPL and then you search the exact one for instruemnt rating. It's mainly straight line distance XC, total distance, daytime, night time, etc.
I'm still not getting your situation. Your heli CPL is Croatian or from other country. Why don't you get it renewed and converted to JAA compliant one?
I can't tell about other countries, or their CAAs, but there's this 'change of licence issue state' possibility, but usually tied to proving that you have lived somewhere past 6 months or so, or majority of year. That way, if you could, IF, swapping to other (EU) country and then have EASA licence once it's rolled over from JAR.
You've got a point regarding the IR training/conversion. But you also have reasonable credits for having IR(A), though not as good as ICAO IR(H).
Wish I had your hours and MI-17 time. That'd be handy for some places/countries.