Is the NIT8665UC covered under warranty if I have it repaired by a non-authorized technician?

No — if you choose to have someone other than an authorized service provider work on your cooktop, the warranty will automatically become null and void. BSH warrants the product against defects in materials and workmanship for 365 days from the date of purchase, but only when repairs are performed by a BSH-authorized service provider. Authorized service providers are independent entities specifically trained on BSH products. (Page 23)