I suspect there is a lot of family tension and a few members have been blindsided, which would explain why they are not showing a completely united front or cohesive logical campaign.
I would speculate that even Tom did not realize how badly battered Jason was when he first came upon the crime scene; so he tampered with the crime scene just enough to take the heat off Molly, then he was blindsided much later by news of the extent of Jason's injuries.
Following this line of thought, if Tom didn't know this was overkill, how could Sharon, who probably never entered the crime scene? I don't believe this was a noisy crime - I think Jason was asleep or half-asleep, getting out of bed and Molly lashed out. So Sharon slipped up talking to the Corbett's and implicated Molly when the party line was to say it was Tom.
Tom and Sharon have maintained an extremely low profile since then - did Sharon close down her FB page because she found out how brutally Jason was injured? Not through any compassion for Jason, but because her husband is now implicated in a more serious crime than they expected. Tom had the option of calling police and saying "My daughter and her husband were in a fight and now he's injured and not breathing. We need an ambulance." Instead he said that he was the perpetrator. Weeks/ months later, Tom and Sharon find out that Molly had used cruel and excessive force, then happily handed the blame over to her father. Speculation only, but I would NOT be surprised if Tom and Sharon had completely withdrawn their support except by not openly talking about the case.
Continuing on this line of thinking, if Tom and Sharon didn't know how bad it was (by this I mean, someone can plausibly die from one blow to the head) and they whitewashed what they did know, then the extended Martens/ Earnest families were getting an even more soft-focus version of the crime, with the self-defense embellishment. Being arrogant loud-mouthed people, they start talking too loudly about Molly's right to custody, probably because they all reek of entitlement, and it wasn't in anyone's nature to meekly accept that the children would be better off with their father's family.
Now all these miscommunications within the family have dug a huge hole for Molly, making the situation much much worse, because they have essentially created a motive for the crime, when if they had faced the truth, they could have argued that her mental issues had triggered the crime.
Speculation continued: Tom's only option is to admit in court that he lied originally, and that he came upon the crime scene and wanted to help his daughter.
Looking at some of the photos here - of the family out to dinner after the legal irritations had been smoothed away, Tom helping Molly with her coat in court - I'm guessing there aren't any more recent photos of Tom and Sharon with their beautiful daughter.