[SalesForce] use $A.createComponent() in the controller

The empty arguments is a bit strange - a side effect of the way your renderer method is invoked by Lightning (Aura Framework) - but if you console.log(cmp) and console.log(helper) you will see that they are in scope and available.

You do not have to use a Helper at all unless you have common code you want to be able to call from other renderer (or controller) methods.

Implementing render(), rerender(), etc allows you to take over the DOM generation and update process entirely or (what I do more often) is to let Lightning markup do some of the work and then custom javascript (often 3rd party libs) fill in the blanks.

Even better in many cases is to delegate all of the rendering work to Lightning and leverage its powerful data binding {!v.something} capabilities with auto update (even bidrectional if you need that). So instead of creating a renderer you can have your .cmp use {!v.something} expressions and just have your client side controller update the underlying model/attribute values and Lightning will automatically (and very efficiently) rerender the subregions from your cmp that are impacted.

I often combine all of the above to achieve the best balance.

This is correct. Please use only one framework (Lightning). If you want to use React, you can do so in an iframe or in a VF page. If we do support React in the future, it would be with a custom component to properly separate it our from Lightning.

UPDATE: I'd like to point out that we may well support React and other frameworks in the future. In this case, please check the Requirements Checklist and our Secure Coding guidelines for lists of approved third party frameworks and secure usage instructions.