Holy moly, you just made my day. I will have to give it a shot. I thought that once all the walled gardens developed their own protocols that it was dead in the water!

All the walled gardens supported in the old days of Pidgin were reverse engineered, same goes for the modern walled gardens. The tools for reverse engineering of protocols (like Wireshark) have only gotten better since then.

Where can we donate to keep the GTK2 interface as long as possible?

GTK2 itself is dead, so you would need to start with a fork of that. For Pidgin, this appears to be the donate page:

https://imfreedom.org/donate/