So following that train of thought, one question is...how did said SK 1) know of Tommy (and refer to him as "Thomas") and 2) get his address? In that scenario, likely from Rachel, and under duress. Or found an envelope in her car addressed to them both and did the math. But I don't know of any SKs who have written so flippantly; they are taunting - either LE or victim's families. They want credit, and to bask in their 'accomplishments' being acknowledged...so writing a fake first person "we had to get away" completely changes the narrative and puts a victim (Rachel) seemingly in control - the exact opposite of known SK psyche.
I think it was one of two events. 1) The note was written by an abductor, or someone adjacent to the abductor/s to buy some initial time before all hell broke loose - trying to send LE sniffing in the wrong direction (Houston), and play down any 'missing' urgency. But it doesn't take a Mensa card to realize panic would've already ensued with a 9 year old unaccounted for before the day was even out, let alone after an all night parking lot stakeout, and certainly long before the letter 'arrived.' You could argue the letter was written before Julie came into picture, but still, Renee is known to have wanted to be back for the party. If the chatter at the scene was "the girls are missing," it's entirely possible Julie wasn't even known to be there until her mother weighed in. The timeline and info gathering stage is snail paced w/o cell phones compared to now. So "girls" would still be accurate, referring to Rachel and Renee and the letter said "we." Whoever wrote it knew it wasn't just Rachel (i.e. I just had to get away", and that's a clue. If it was only based on her car, then it had to have been someone who knew Renee was in on the plans beforehand, saw them at the mall or heard one way or the other that Rachel wasn't alone.
The 2nd scenario is a shortlist entry for the most ill conceived hoax of all time. As in, completely unrelated to the actual disappearance, just some kid being a d. So in the moment not thinking, "this is hysterical, 50 years will go by with no answers, I'm such a hoot." After days/weeks deciding to not come forward and risking whatever consequences a kid would imagine. Or at the very least avoiding the reaction of the families if there were any remorse. So, with no internet or even CNN yet, that kid goes to college on the east coast, moves to Canada with family, ODs on vacation in Galveston, gets hit by a bus, who knows...it's entirely possible it was forgotten about. A triple disappearance is unusual, but in '74 into '75 not front page news across the U.S. and if it was, not for weeks and weeks as would be the case now. Coupled with LE who while might not be involved, sure as hell don't want to advertise they have no answers AND the fact that LE was woefully disconnected to other jurisdictions, it's a mess. It seems inconceivable to us WSers that someone could not have heard about this case. But IMO 99.9+% of people likely never have.
One thing that's always given me pause is the language. I've mentioned it before, but was someone like Rachel (or anyone for that matter) likely to say "catch it"? Is that regional, at least in the early 70s? And "we just had to get away" sounds so airy, and non-teenager like. Almost like it was pulled from a Virginia Slims print ad or something someone's mother said a lot. I suppose "blow this popsicle stand" and "gets to steppin'" have their timeframes too. But it's also important what the note DIDN'T say i.e., "eff all y'all, we're never coming back" AND simply stated the obvious..."the car is in Sear's (sic) upper lot." Who wants the car to be located asap, and why bother? Unless you know there is no way to tie you (the writer of the letter) to it, because you weren't in it, and likely didn't touch it.
