mobile Safari is embedded in iOS. It can be deleted by jailbreaking and gaining root access, but removing it would cause iOS to malfunction. You can't delete it entirely from iPhone, but you can effectively delete it by removing it from the Springboard, entirely disable it in Settings (using Content & Privacy Restrictions, toggle the switch next to it in Allowed apps), and make it undiscoverable.
And although all web browsers on iOS use WebKit, there are effectively only two browser rendering engines still used today, WebKit and Gecko. And only FireFox uses Gecko. Every other browser uses WebKit or an engine derived from WebKit. So while Apple considers allowing the other rendering engine,[1] Gecko, I guess you're stuck using any other browser other than the official Gecko version of FireFox. There's a WebKit version of FireFox for iOS, if you can stand to use it.
You are incorrect. Blink might be a fork of WebKit (which in turn is a fork of KHTML), but they are definitely not the same; WebKit is generally inferior.
Thankfully, the EU is forcing Apple's hands, and soon we'll see better browser engines running on iOS.
This is not possible on iPhone though. You can't even run any other browser than (reskinned) Safari.