If you read the preliminary hearing, the prosecution themselves says they don't deny others were/could be involved, just that doesn't change the guilt or innocence of DW and also LE has previously said others were involved. I view it that way too where at best DW was like a getaway driver for a bank robbery where he didn't know or intent for a bank teller to get killed in cold blood, but would still be guilty of felony murder anyway.
If DW was leaving hostages all by themselves - as testified by the prosecution witness that DW was seen arriving on foot on the 14th - that would have been extremely risky as by the time he got back to the house, the hostages could have escaped or otherwise create an alarming situation. DW couldn't just say there's someone downstairs so don't do anything nor give free access to cell phones while he's away, so if someone showed up at the door, the Savopouloses would have waited for prompt to respond and if they didn't get a prompt from an imaginary co-conspirator they would have remained silent causing the visitor to panic or if they knew they were alone, they'd straight up tell the visitor that they're being held hostage. Whoever was involved was clearly controlling telephone access and wouldn't allow SS or anyone else to freely use their cell phones, if not some of the messages actually sent by whoever controlled the phones. This also potentially puts DW not even being at the house when the money was delivered.