I have now run four tests on four different pairs of my shoes. I have gone for a walk in each pair for either one hour or two (depending on whether the tide was too high to walk on the beach.) I've noted whether there was any pain on each walk.
I walked in Chaco sandals, Feelmax Osmas (very minimal moccasin-like shoe), Brooks Cascadia (trail runners that I wore on the PCT, a new pair) and my new Piper sandals.
The Chacos have a bit of a heel rise and a strong arch. The Piper sandals have no heel rise and a small arch. The Feelmax are completely flat without any support at all. The Brooks Cascadia are typical trail runners but they're not motion control shoes.
I limped and felt a lot of pain wearing both the minimal Feelmax shoes and the chunky Brooks Cascadia. My sesamoid injury hurt, my achilles tendonitis hurt, it felt like my left foot was rolling inward, behind my left knee it hurt. I limped because it felt like my left foot was falling into a crater in the Brooks. It really bothered me the way the ball of my foot sunk into a cavity with my toes pointing up. In the Feelmax there was some pain on the ball of my foot where a bone seems to crunch there.
In both of the sandals I had none of that pain, except maybe a little bit of the last one, with the bone on the ball of my left foot. There was no limping, no foot turning in, no sesamoid pain, no achilles pain, no pain behind my left knee. No feeling like my left foot is falling into a crater. I felt straight and true.
I don't know what the secret is. Is it the freedom the sandals offer for my toes? Is it the lack of EVA foam? Is it the relatve flatness and simplicity of the footbed? Maybe I don't need an insert for my shoes, maybe I just need to stop wearing shoes and wear sandals instead. I'd like to see if I could get a pair of custom shoes for hiking. Something that truly fits my feet. Maybe my problem is they just don't make shoes shaped like my Hobbit feet.