I have used regular gas in place of premium several times over the past couple years in my 300e. Usually on long trips where the stations don't have premium. I try to use as little regular as possible and top it up with premium again asap.

I have never had any rough idling or poor performance with regular gas so I wonder if your husband hs tinkered with something else under the hood...just kidding. I have never heard any pinging with the regular gas either so either the knock sensors or the engine soundproofing is doing a good job.