Sorry to hear about the issues, it can be terribly frustrating.
Most of the issues I've seen seem to be related to either the clutch actuator or the shift motors. On mine the shift motors failed because the brushes on the two electric motors wore out. I still need to get motivated and buy replacement bearings and brushes to rebuild it.
I don't know what year yours is, but being a 451 it's certainly no longer new whatever year it is. With the loss of the Smart Centers and the general lack of quick turn around on parts, they wouldn't be my first choice for a car someone needs to depend on. Not knocking the car, they're as good as any other, but the support network isn't there like it is for more common cars. I think they're still great for tinkerers and shadetree mechanics, but probably not so much for your average car owner.
If you get to the point where you're done with it, one can usually get a bit back selling it off as a parts car. Prices vary depending on demand and where you live. It's always a hard call and every situation is different so I can only advise to keep the sunk-cost fallacy in mind when evaluating what to do. Much as I like the Smart, repairs can easilly exceed it's value on older models.
Best of luck finding a workable solution and if you decide it's done, please consider trying to sell it here first.