Questions (SPOILERS)

If memory serves, Oliver did not kill Farleigh, the grasping nephew of Sir James. How can he depend on Farleigh not going to the police to investigate the circumstances of his aunt's coma and death? Wouldn't Farleigh also delve into the circumstances surrounding Sir James' death / his will? In other words, Farleigh would be the heir of Saltburn after Sir James, his children, and widow all die? If he were to see that ultimately Oliver inherits Saltburn (and basically the whole Catton estate) wouldn't he and his mother urge the police to investigate Oliver and hire an attorney to look into an undue influence case?

Also, are we to believe that Oliver killed Sir James and made it look like suicide? The denouement was so rushed, it was hard to determine that which Oliver did and that which he did not do?

Finally, are we to believe that Oliver and Felix and Venetia's mother had a sexual affair and that Oliver married her? I have to say, I was so bored up until the rapid fire reveals at the very end, I may have missed some of this. But in the interest of completeness, I'd like to fill in as many blanks as I can. Make it seem those two hours weren't an utter waste of time.