Why do you want to do these things? There might be an easier way.
Yes, this should work fine. Switch on DB_SEE_INPUT in formmail.php to see the result so you know why you're getting the no_recipients error.Is it possible to make the required 'recipients' field a "derive_field"? ie
derive_field = "recipients=toEmailUser + %40% + toEmailAddress";
I'm pretty sure the derived field would overwrite form field.What if I included the recipients field on the form AND derived the field? Would the latter overwrite the former? (Try it and see he says.)