Expanding the courts doesn't solve any long-term problems. It's short-sighted, at best. Term limits just makes things swingier. At any point in court history, having a 6-3 division among left-right lines has not been unusual, and it goes back and forth.What would be wrong with expanding the court to 15 or 21 justices, creating term limits (maybe 10 years? or 15? I'm not set on any particular number), and drawing names out of a hat (or doing some random 9 or 11 selection from the pool of justices)? What would be wrong with this? I think it solves a lot of problems and it diminishes that incentive that Republicans currently enjoy via legislating through the Judicial Branch.
As far as winning back the courts, that's one hell of an uphill battle. As you stated, Voters may not care. Also, the Federalist Society is so well funded and organized. The left doesn't have anything close to that. Then, there are the built-in advantages of the EC and Senate that Republicans currently enjoy.
Left-leaning jurists can easily build their own version of a Federalist society if it's needed. but I don't think it is. The natural bend of law interpretation is left-leaning (AFAICT), and the Federalist society was constructed to fight that bend.