Sorry for just coming with it out of nowhere, cause I haven't read the full discussion yet, but kept stumbling on this case mentioned here and there.
As for now thou, I still haven't stumble on maybe bit ridiculous thought but it keeps coming back to me every time when this thing with "Rachee" corrected to "Rachel" is mentioned.
I know about a lot of writing analysis happening with comparing Rachel's handwriting to the one in the letter - but was it ever compared to Renee's handwriting?
I always wondered - was that person writing the letter trying to keep calm while under tremendous stress (like being forced to write it) and, if it could be Renee trying to write as Rachel?
With some odd scenario like: girls met someone they knew, went somewhere, at some point Rachel got separated from them and Renee, as her friend (so as the one likely well aknowledged with her handwriting) got forced or convinced to write it (with someone saying something like: "X thing happened, Rachel had to go to the Y place, will be back after christmas, we had to help her, we will join her in a bit but to explain stuff to her family and not make them worried you (Renee) have to write this card").
If Renee's handwriting was totally different from Rachel's, then either:
a) Rachel wrote it on her own - cause with whatever reason girls indeed needed to "get away for a week",
b) Rachel got forced to write it,
c) Rachel got convinced that she has to write it to somewhat explain their absence but the message in the letter had nothing to do with - what she believed at that moment - were their actual plans,
d) letter was forged by someone with access to Rachel's handwriting,
e) text part was actually written by Rachel, at some point in the past, when "we" didn't meant her, Renee and Julie but her and someone else, and that typo in her name indicated nervousness and got thrown away by her and used by someone who well ahead planned to abduct her.
This is all far fetched, but facts here are quite strange. For me it looks like someone was hard set on kidnapping/luring Rachel somewhere on that specific day, so hard set that it didn't matter who she was with. Like with whatever reason that was the only time when it could be done and other two were too dangerous as possible whitnesses to let them go.
Sorry for sharing these likely rubbish thoughts, but I'd like to see how my theory will change as I'll learn more about the case.