Will Apple reject a submitted app that has a combination view using WebView and native In App Purchases?
The idea is that the WebView will display the benefits of the premium service and include the price plans via an external website, the native UIButton on top of the WebView will allow users to complete their transaction natively using In App Purchases provided by Apple. This way I can dynamically test different price plans and designs without the need of updating the app. The website will send webkit callback handlers to communicate with the native app in order to make the purchase of the selected price plan (IAP product).
No one can definitively answer questions that begin "Will Apple reject an app because..." Their review guidelines are constantly evolving, so take all of the following with a grain of salt.
What you are doing is technically possible and doesn't seem to violate any of their rules. You are still forcing users to complete their purchases through IAP, you are simply using the Webview to supplement the IAP details. In fact, there is a company called SweetPricing that does what you suggest. It is a paid service, so it may not be worth it for your experimentation.
A simpler solution would be to have multiple versions of the same IAP that have different price points. Then in the app, you would offer the different price points at different times to test your pricing model. You can find details of the approach here.
Your native component will still need to trigger the native IAP prompt, but you certainly can use the webview to use one IAP over another.