If the MCAS is triggered by one sensor, than simply adding another won't be enough. There needs to be three just like having a standby A/H as a tie breaker.
Surely any mandated modification will require more than a simple software update.
Concorde was grounded for a lengthy period after a design flaw led to a disaster. Extensive modifications were required to the wing tanks before the aircraft was deemed safe. Any solution proposed by Boeing will need to be very conclusive in restoring safety for it to be accepted and it won't be quick or cheap.