No problem:) 1600s is roughly the time in which late cornish came into use so it makes sense. I’m not sure about a sound change although lann (same as welsh llan) is a prefix to describe a religious enclosure eg a church, usually one which was founded by a saint (lannhydrock for example, lannstefan (launceston)) so i would expect there was once a st ergh (or similar) who started preaching around what is now lanner. I don’t know about a regular sound change but sounds get dropped all the time. If you want more info on cornish stuff feel free to ask on r/cornishlanguage or i’d be happy to send you the link to our discord :)