I want to run 2 breakers with 1 run of 14-3 up to a junction box then break off from there with 14-2 but can it work using the ground and neutral for both circuit runs?

Yes, but breakers have to be a double breaker. In other words the handles of the breaker have to be tied together.

Yes it can work fine. Thing is, if this is for most circuits in a house the circuits will need to be arc-fault protected. There are very few double-pole arc-fault breakers out there. Especially for an older panel.