You are the one who has to define what your product is and how it will be designed and produced, the test house won't do that. So they only care if the product lives up to the EMC criteria or not - they don't care how you get there. As for what's acceptable for a certain kind of product, that's a very big topic.
Should you fail an EMC test but do something "ad hoc" while in the lab (hello, ferrite beads) in order to pass the test, this patch will be documented in the report and the test engineer will need to take photos of it.
From there it is your responsibility to produce the products the same way as they were tested. If you release a product on the market and it causes EMC problems, you are in serious trouble in case the production units are notably different from what was tested and you can't present a sensible argument regarding why. For example by presenting a product log explaining changes made after EMC testing and why you believe that each change done does not affect EMC.
Would you say that it is acceptable industry ptactice to later on in production replace the foil with metallic part or spray without re-test?
Obviously not since this is a change of an important EMC component - you cannot sensibly argue and say that it does not affect EMC. And from my personal experience of such sprays, they are of questionable quality at best and do not work well over time either.