The JCAB doesn't look too bad. You only have to do Air Law differences and the JCAB Radio Licence. You're 6 months timeframe is inclusive of the full type rating ground school for the Aircraft you fly in Japan. I would imagine without it then it would be a lot less than 6 months just to pass those 2 exams.
If it was just a case of an Air Law differences exam, and a bit of time in the sim to satisfy JAA then it would be a walk in the park. Sadly, it isn't and this whole process is a lot longer, costlier and time consuming than the JCAB example you give. It also means zero to me as I don't legally require JAA licences to fly an American reg jet. It's a totally different situation for example than working in Japan and flying a Japan reg jet.
I haven't looked at your other example in depth but I do know that it is easy and straightforward to obtain a GCAA ATPL.
Don't worry about us getting impounded, not going to happen.
And just another thought for you MJ. I don't know what type of flying you are currently involved in, but why not give "N" reg a try.
You'll have the added benefit of better trained crew who get proper recurrent training, rather than the naff LPC that my granny could pass.
If you want any pointers, then I'm sure any of us who fly "N" reg can help you. Your boss may like the idea, are you Aberdeen based ?