Thanks for doing this Peter! I am an Indian national on H1-B with an active, non-withdrawn I-140 and a Priority Date of August 2016 from a previous employer. My current employer has an approved PERM, but their I-140 is still pending.

If the Dates for Filing (DFF) chart reaches my priority date, what is the latest viable timeline to return to my previous employer to file an I-485 based on their approved petition? Additionally, are there legal risks to joining after the date has already become current?

As long as your PD is current per the DFF chart in effect, then there's no date by which you must file your I-485. It's different if the Final Action chart were current. In that case, assuming it remained current, you would need to take I-485 or IV steps within one year. Of course, if you return to your current employer, it would need to be a bona fide job in the same/similar position.