How would you quantify ongoing projects where workers come and go and each of their specific contribution might not be easy to measure?

Probably some sort of collective ownership, profit sharing, with negotiation and consensus building. Other people more well read than me have spent a lot of time thinking about this. My starting position is that the standard capitalist model of "I pay you $10 to make a widget, and I sell it for $1000 and keep all the profits" is not okay.

Do they all also assume financial responsibility for any failures or lawsuits?

Do the owners assume financial responsibility now? I think that's what LLCs and other corporate structures are for- to shield individuals from liability and responsibility.