Is it possible to set the value of the To Be Emailed field on transactions via workflow? I already tried it after field sourcing, after field edit and before record load but none of them works. Only before record submit but I want users to be able to override the default value.

If what you say is true, it feels like NetSuite is kind of using the before load to set the email themselves? So in this case you need to work within the before submit or before save area. You could create a new field (type free form text) and have the users use it instead. Then before save you move those values into the actual “to be emailed” field. You’ll need to validate the data entered into this custom field, because the “to be emailed” field can accept multiple emails and your free form text field is not going to automatically do any validation. You could make the custom field be type “email”, but it will only take one email.

The Email Address field is okay. It's the To Be Emailed checkbox that I can't set the value of unless I use Before Record Submit

If you set the email preference on the customer record it will check the box just like using before load

But if I do it on the customer, all transaction type will be marked as such. For example, for a certain customer, I want it ticked by default for sales orders but not for invoices and credit memos.

Do the same idea, a fake checkbox for users, you use that to manipulate the real checkbox before save.

Even the fake checkbox is not being populated. I have turned off the other scripts and workflows. It's so odd

If even a custom checkbox is not being populated, it sounds like the workflow logic is possibly setup incorrectly. Have you checked the workflow logs? System Info -> Workflow History -> Log. Should show you all the steps your workflow went through and maybe it's not validating something to be true/false? I usually default to scripts so that's about all the help I can probably give on workflows.

I don't see any logs related to client triggers.