[Facts] Re: The use of Evelyn before Pierrepont
In the UK, Leslie Dunkling wrote the Guinness Book of Names: definitely designed to be read by people who don't need to know the difficult details. Scholarly but scanty. Anyway, in the chapter First Name Origins: The central stock of girls' names, he lists "Evelyn: historical; Old German; of uncertain meaning." Unfortunately, that's all: no suggestion of how and when an Old German name became English.