OK: SOLO performers: Arthur Rubenstein

Bands with membership changing: dunno-- maybe the Bluegrass Boys? (1941-1996)

Bands with constant membership: the Six and Seven-Eighths String Band of New Orleans who (according to their Folkways record of 1956) first got together in 1914 and lasted till at least 1960 --46 years!!

Solo performers in country/oldtime/folk, probably Wade and Julia Mainer

didn't George Jones make his first Mercury records in the early 50s??