If the fire-starter was a guest, more attention would be drawn to a guest walking on the grounds with lit lamp in hand than the help
The only guests we know (i.e. characters rather than extras) are Liam, Mrs Daniels and the Colbys (I don't know if they were invited but the were in the mansion shortly before the ceremony).
I'm not sure if the staff and help were paying much attention to the guests, if you consider how easy it was for Claudia to walk into the mansion and then upstairs.
Anyway, which one of those guests would have a motive?
The Colbys, because they felt betrayed? They seemed very pleased when Hank voted with them to sell C-A. When Jeff suggested another partnership to Fallon I believed him.
They're practical people, I don't see much emotion coming from these Colbys.
Melissa Daniels? Because...because...??
Liam? Anders forgot to un-invite him which is an odd excuse. Why would Anders invite him in the first place? If the invitations were made at the time he was still married to Fallon he didn't need an invitation because he was part of the family. And if Anders had sent him the invitation after he had left, then why would Anders change his mind and un-invite him?
But maybe it was just a joke and he invited himself.
His motive would have to be fabricated, like a whole new background story completely unrelated to anything we've seen in season 1. Would the writers already be that desperate?
The same goes for Anders, there's never been any indication that he hates the Carringtons.
It's more likely that someone infiltrated the group of hired help, and that could be Cesil or Ted Dinard, although my guess would be Ted's father.
It's all good and well to create a mystery, but if you explain it with a nonsensical out-of-the-blue motive then it falls flat, to put it mildly.
but the arsonist was outside when he took the lamp
However, he/she must have been inside that part of the stable house first, to sabotage the door with the screwdriver and then go outside to get the lamp.
Anders was talking to Cristal, then left the building, Claudia entered the building and after her the Carringtons. Claudia shot Cristal and then left, she had the opportunity to sabotage the lock but that would have been pure improvisation as she was too unstable to plan anything like that.
Or maybe it was one of the photographers

, but they didn't know that they were going to be excused. I assume they also left the building otherwise they would have seen the person with the screwdriver. Come to think of it, they must have left the stable area altogether otherwise they would have seen the person with the lamp going inside the building.
Unless...they never left the building, which means...
What are your thoughts on how burned-beyond-recognition body gets there
This is more complicated than Who Shot JR.
