I think the big toe is relevant ... there is a certain strength exercise for the plantar tendon which will include scrunching a towel under your feet .. so therefore if curling your big toe causes discomfort then this would back that up
A lot of achilles/plantar problems will come from tight calfs, and further up the leg and you are right to work on releasing them, did the sports massage bring any tightness to light ?
As you will know this is a tough old tendon connected to a tough old tendon .. it takes time to recover and will take a lot of stretching and releasing
As to if it is connected to the shoes, why not after a period of time try the other shoes and see if the problem persists
With my achilles issues I have found that ice massage works well along with stretching and massaging