And on a plot structure level, showing Marta and the medswap incident in like act 2 combined with the general genre almost certainly dictates that she isn't the ultimate culprit
(The one I seized onto was "the dogs won't bark, they know you" vs. Meg waking up to the dogs barking and when Ransom was properly introduced with the dogs barking at him I knew)
Ransom: [angry rant about Marta getting the inheritance, the house being his ancestral birthright and so on] Blanc: Please, your grandfather bought this house from a Pakistani realtor in 1980.
apparently Rian Johnson also sneaks a Joseph Gordon-Levitt cameo in all his movies and he's impossible to find in this one unless you know his voice really well
and yeah, the Patriarch essentially explaining the solution reminds me a lot of Three Coffins, Blanc calling Marta "Watson" seem to be a call out to the Murder of Roger Ackroyd
Blanc: Please, your grandfather bought this house from a Pakistani realtor in 1980.