One of the overriding principles of the CAR and SoGA before it is that nothing can overturn your rights under the Act. Manufacturers wording should contain a phrase like " Your statutory rights are unaffected"
The point is your contract is with the seller ( not the manufacturer);
If the seller referred the job to the manufacturer, then legally the seller is subcontracting the job to a third Party, but also legally they cannot absolve themselves of liability as they have contracted the work and should have used due diligence in choosing the best supplier to carry out the work.
If the work is substandard the seller is responsible because they should have engaged a subcontractor who would carry out competent repairs. The fact it was the manufacture has no relevance.
The CRA has to be used properly and that does mean it may not be the most convenient process, but it is there to help prevent consumers being taken for rides by sellers.