If a married couple on L1 visas both comes in to work in the US and one loses their job - can that person switch to the dependent visa, then back to the L1 visa when they find a job? Would the green card wait be reset then?

The spouse could definitely switch to L-2 but then might lose eligibility for L-1 since the spouse still would need to meet the qualifying foreign employment requirement and the three-year lookback period for qualifying employment abroad would start now, not when they initially transferred to the U.S.