Now her "missing years" become a bit more mysterious. I say this because I would expect her to leave PA, drift a while until she tired of that lifestyle, then start the process that ended with her recreating herself as LEK. If she'd been living as an "urban nomad" (as I heard one person who lived that way describe it), she'd have picked up the knack for finding someone to crash with -- which appears to have been how she was living in Idaho & at first in Dallas.
Or what Kafkette suggested in post #1118 in the thread "TX - 'Lori Ruff', Longview, WhtFem UP9863, *General Discussion and Theories* #5", that KM had become part of the L.A. model/actress scene for a while, & when it turned toxic -- Kafkette mentions the Wonderland Murders -- KM fled. (Kafkette points out that KM could have been at the very edge of all this.) Stories of young people running away to find fame & fortune in Hollywood, only to discover the ugly truth are so common as to be a tired cliche.
But you are right about the short time frame for her "missing years". If she was 18 when she broke all contact, that would have been late 1986; according to S/S1988, she was in Dallas by spring of 1988. I don't know your source for placing her in Devon in 1987; if it was a phone book, its date does not indicate when she was living at that address, but a few months before. (At the time, it took that long for phone books to go from compiling the entries to being printed. In 1987, I had a temp job proofing ads for US West, so I know a little about that process.) So she period might be as short as 9 months, or as long as 18.
But to understand this blank spot in her life, I'd start with any memories of her former classmates in PA, & information there. Maybe KM was a promising student who fell apart under the presence of a step father; maybe she'd always been a bit odd; or more likely, something I haven't considered.