It used to be possible to get a UK ATPL, which restricted you to G-registered aircraft, by passing only two exams (Air Law was one but I cannot remember the other one). But that option has disappeared with EASA. A validation of an FAA ATP is for one year, with an additional one year re-validation. But after two years you will need to have passed all 14 ATPL exams to be able to still fly in Europe. Also be aware that EASA has proposed a rule where a validation of a non-EASA license will be restricted to aircraft registered in the nation that gave you the validation. The rule is not in effect now, but when/if it does, it will severely limit options. Also part of the rule is that you can only have one validation at a time, so you will not be able to get your FAA license validated in the UK and Germany at the same time.