Opera-mini is available, probably because of some media/industry pressure but also because it uses a proxy in the middle and a different protocol (i.e. not HTTP/HTML) to deliver content to the client.

It would be more correct to say that all web browsers have to use the Webkit as the rendering engine. There are actually several web browsers available, most to add better functionality like real tabs, searching on pages, an option to use Google's Mobilizer, etc. and others e.g. to provide parental controls. One I use (mainly for tabs and in-page searching) is Mercury.

Not sure how much control developers have over displaying content, dealing with font size issues, etc. when using the Webkit APIs. Seems like most developers like to punt that stuff over to Google Mobilizer.