Will Linden Lab's choices make them legally culpable?

As we are assured, time and again, age/identity verification will be totally, utterly, completely, and in all other ways optional. You don't have to do it. You'll just not be allowed into an adult-flagged parcel if you don't.

Flagging a parcel that contains adult content is mandatory, and that's where the trouble might start. If Linden Lab investigates and enforces, essentially, editorial control (especially since the guidelines for what is and what is not flaggable are fuzzy) then they can be considered to responsible for what gets missed, in some jurisdictions, possibly defeating common carrier/service provider protections.
This sort of thing can still be trumped in some jurisdictions by duty of care, but the specifics of that are narrow and might generally only be applicable to Linden Lab on the Teen Grid.

Nonetheless, it seems possible that the adult content flagging may (depending on Linden Lab's policy to how it is implemented, investigated, and enforced - things we do not yet know very much about) land them in legal hot water down the track.