1) His dad in fact did practice with/own this firm, and may retain equity rights even in retirement. This means my unilaterally imposed moniker, all along, was more accurate than we could have known! I have always called the firm Manageing & MANAGEING LLP, and sure enough we know now that it is indeed a father-son thing with (likely) a repetitive name, and nobody else's in the mix whom we have heard of.
2) MP's Dad is now in his nineties. This provides a potential clue as to MP's own age--a generation apart would put the Junior son around age 60, which is what I had thought. HOWEVER, since MP currently has a much younger (fecund) wife, it's possibel that his Dad did the same thing, marriage-wise, years ago. In that case, MP Senior could have been 60 when MP was born, allowing the son to now be more like early middle-aged. I consider this question open, but again, especially with MP having a nephew in his mid-twenties, I think chances are better that he's old, gross and bald as opposed to just gross and bald.