So with great trepidation i took apart my steam deck, replaced the shoulder button, and managed to reassemble it back together! And everything works! ... At least as well as it did before.
Pretty sure the part i needed to replace is the button circuitry and not the button mechanism. And it doesn't seem like the part i need is available, boooo.
So now i either get used to the defective button, pay $150 for Valve to repair it, or just buy a new one. Hurmph.