Person A, a German citizen who currently holds a US green card and lives in the US, worked earlier in their life in three different European countries, contributing to various government-sponsored pension plans, from the late 1970's to the mid nineties, with a little hiatus in the middle, for a total of 15 years' worth of regular contributions (Germany 9 years, Denmark 2 years, and France 4 years).
Person A more recently contributed to the US Social Security retirement system for 20 years (and participated in an employer-sponsored 401k program -- although that has nothing to do with this question).
Can Person A receive benefits from both the European pension system and from the US Social Security retirement system? Will the US "Windfall Elimination Provision" (WEP) affect things?
Should the three European pensions be somehow combined or centralized, perhaps into the German one?
Should Person A consider making voluntary additional contributions to the German pension fund?
Would receiving US citizenship somehow affect things?