Great investigative pics and the older one looks like a copy of a traditional steel BB shell set up... but being Ally has obviously worn more with cables.
I wonder if that nasty crack could have been caused by vaulting/landing on a log?
Or I remember that I bought a clamp/strap-on chainset bash-bar thingy (luckily for my previous Overbury’s Crossfell steel bike) that was designed to protect the expensive BioPace chainset teeth, but would actually have transferred shock to the underside of the down tube (scary add-on accessory on reflection).
Thanks for sharing though.
I’d still say glued not screwed and glued for carbon tube though !