]Parrish is a very suspicious character, too (IMHO).

Not only Parrish. Mr Kendall who seemed a mere nuisance before has become one of my principal suspects now (along with Parrish and Randolph).
Kendall could have manipulated the carriage somehow or have ambushed them on the road. He arrived just in time for it.
And he seems so determined to bother Mr Bingley all the time. This cannot really be about this old business, I think - unless the cargo plays an important role as someone has indicated.
And when the fire breaks out he is fully dressed and searching for the business papers. Perhaps he found something and started the fire to distract from his own doings or to veil that something in the house is missing (that he has stolen).
Or do you think the fire has been lit to kill the Bingleys like the carriage accident? I rather tend to think that its main purpose is not to harm the inhabitants but to destroy something. But what? There must be a reason for this fire as must be for Caroline's riding accident, the attack on her in the kitchen, the Bingleys' carriage accident but I can't see it. What can be the connections between these incidents? That the victims (the Bingleys and Caroline)are related to each other? But if so why has Louisa not been attacked yet? Too many questions; I better stop now before I drive myself mad ;-)