If Mark Alexander killed his father, removed the body to another location where he attempted to burn it before returning it back to the family home for burial then he used a vehicle to carry out these movements. Consequently the claim that had he used a car or some other means of transport he would have been seen falls by the wayside.
There is no evidence for this at all. It would have required Mark to for example, rent a vehicle, and that would leave a record trail. No such trail exists, because he didn't use any other vehicle. We agree with you though, it makes sense that whoever was reponsible used a vehicle. It just wasn't Mark.
We can all sit down and think of alternative explanations when the evidence doesn't fit the current theory, but this doesn't get us anywhere without proof. The risk we take by doing this is falling into the trap that the police fell into, which is one of tunnel vision, fitting the evidence to the suspect. But when all the evidence starts to fall apart, instead of trying to make up some other theory, shouldn't we instead be asking whether we might have got the wrong guy in the first place? The refusal to accept that a terrible mistake may have occurred here seems worringly familiar, it's exactly what criminal justice agencies have done time and again in the worst miscarriages of justice.