Your explanation is plausible, but barely. I haven't known many mothers-in-law who would go to bat for a son-in-law. That's more difficult to understand than Cynthia Anthony's being in denial about her own daughter.
I don't know. I've never stood on my front porch and had the police ask me questions about where my 17 year-old married child might have disappeared to along with a 14 year old "friend" and a 9 year old stranger.
I do think that Tommy wrote the letter, which is difficult to explain if he didn't have some culpability.
Not hard to explain if he confesses that he wrote it to make her family calm down. According to him - maybe she had threatened to pull a stunt like this. As long as he confesses to the letter, he's just stupid. If he denies writing it, he's screwed.
Still, Cotton seems like an über-creep who hung around with other über-creeps. His possible involvement should not be dismissed out of hand. It definitely seems like there's something rotten in the state of Arnold.
Motive?
If Tommy acted alone and drowned to girls in Benbrook Lake, as you believe, he would have had to walk all the way back to the bowling alley, right? Could he have walked all that way without attracting attention to himself?
Absolutely. I did, January 5th, 1968 - after dark.
You've speculated that the army corps of engineers might already have pulled the murder car out of Benbrook Lake; perhaps a FOIA request could get you their records (probably not free of charge).
I have a simple plan.
Are there other bodies of water that might be possible grave sites? A much smaller lake or pond would work; many farms have artificial irrigation ponds deep enough to hide a submerged car.
Water, water, everywhere. This lake at that specific location fits Occam's Razor; the principle of parsinomy.
