@eva

You should be able to use the Syphon output from stage setup to allow OBS to grab the input. 
Using OBS is a good choice for big video like this, it allows completely separate cpu/gpu processes to deal with the video recording, which will ensure Isadora provides the best possible playback
Yes, set OBS to record at the speed you set the Isadora framerate at.
Since you are on Mac you could try Syphon recorder, it may be the easiest option. Syphon Recorder | Syphon

@tomthebom said:

reexporting

 When video files are acting funny, this is the answer 99% of the time.

DusX and Woland, thank you very much for your help and useful recommendations!

A special thanks to DusX for suggesting that we bundle parameters into JSON and for sharing the guide to JSON Bundler and JSON Parser. This really helped us understand how to reduce the number of inputs and outputs and simplify the connections in our project.

Woland, thank you for testing this in Isadora 3.2.6 and for suggesting the additional options using Broadcaster / Listener and Set / Get Global Values.

Regarding our statement about previous versions — we were mistaken. Thank you for checking and correcting us!

We really appreciate your responsiveness and the time you took to help. Your advice has genuinely helped us move forward!