Nickel allergy

For the watch band it will depend which band you have selected. If the protection product is solvent based it might damage the band so be careful. You could consider one of the bands like Solo Loop which don't have a metallic component.

For the back of the Apple Watch I can't give a definitive answer, but it is possible a protection product applied to the watch could block the optical sensors used to measure heart rate (and blood oxygen in latest models).

If you're concerned about having nickel touching your skin, would the Stainless Steel Watch be better for you? I don't believe there is ANY nickel in the center of the watch which houses the sensors on the bottom of the watch as that part is ceramic. And the connectors for the watch band honestly don't touch your skin at all anyway.

Worst case, if you buy an Apple Watch and it causes an allergic reaction, I would hope you'd notice it fairly quickly as you have 14 days from date of purchase to return it for full refund.

I would also hazard a guess that applying any solvent on the watch itself, could void the watch warranty.