I would probably queue up the background job and pass the jobs record id. This can manage the state of the background job. Once the job finishes, it can attach to this record the zipped file. It can then send out the notification and the user can download the finished zip.

You could create a PORO, view component, helper or partial which has a generic modal. From there, you could pass in the locals required for the title, content, etc. Personally, I don't mind creating a new modal for this kind of stuff, but I do see the benefit if modals are a primary way that users interface with the application. 

You shouldn't have to run the precompile to get it working. Are you using esbuild or importmap for your application?

I haven't come across that error, but if I had to guess, it could be something to do with this line

event.target.closest(".nested-fields")    

And it not being found. Do you have any more info as to where the error is located?

In these kinds of situations, I'd inspect the HTML elements to see what's getting rendered as it could be a bit different when using SImpleForm.

I would also hop into the browser console to try and target the element to see if I can retrieve it.

document.querySelector("input[name*='_destroy']")