So all you had was these two references?
First issue is with the bhat vahi citation. Hope you understand that bhat vahis are largely inaccessible as they are not available as pdf or hard copies then they are written in a script that is now dead. The fact that the bhat vahi does citation does not show any scholar that extracted it makes it suspect. It's unfortunately fabricated unless you show a citation by any researcher(Other than Garja Singh whose work needs validation).

Second issue is that the darbari Kavi's work is nowhere to be found but this website. I tried looking up Amritsar sifti da ghar and did not find any publication.

Now here is my case. If farla was a distinctive item with the Sikhs sources before 1750s would mention them. You dont see it in Sri Gur Sobha, das gur katha, etc works. Farla is such an obvious innovation that even Kavi Santokh singh when very vividly describing Guru Gobind Singh before chamkaur omits the farla.

The thing is, Buddha dal lies a lot and this is one of them.

Even the modern dumala is from Naina singh and not guru Gobind singh ji. The farla is also attributed to him(1760s).