Not for a moment. The more convoluted Mike's stories become, the more impossible to believe they become.
When you eliminate the impossible what remains, no matter how improbable - must be the truth.
It's
impossible that Sheila killed the family (too clean, no energy, quite frail, never handled the gun before .......)
It seems improbable that Jeremy would tell his girlfriend of his plan to kill the family, carry it out, then dump her, that he would try to pass off two shots to Sheila's throat as a suicide, invent a hit man story, show no concern for the family tragedy (especially burning his parents clothes in the farm fire pit), try to sell explicit pictures of his dead sister and go on holiday at the drop of a hat. I could go on ..........