Lots of plausible theories, but frustratingly little to go on. We do have a party with an alleged motive, documented financial strain, alleged impropriety, knowledge of the garage sale, and up-to-the-minute involvement in Sergios work schedule, vehicle, etc... that remains the leading contender in my book. when(!) an arrest is made, I'll be somewhat surprised if this person wasn't involved. (moo!).
I wish Pat Brown discussed that angle but in her video she didn't even know the husband's name let alone other suspects named. Someone with financial issues and an insurance policy does point to a STRONG MOTIVE. But according to Sergio it's 500k but he did not seek to request it.