[SalesForce] Email Tab in Lightning Experience for Opportunity object

I don't think it's possible right now with GA functionality. Your only option might be Lightning Extensions, currently a pilot feature. Or, for a much higher LOE, you could build a Visualforce page for an action override.

Summarizing your question:

1. You have 2 record types (say A and B) on Opportunity
2. On a Lightning Component, if a User selects A, they should be navigated to standard Opportunity creation screen
3. If a User selects B, they should be navigated to your custom lightning component

As in your current implementation, it is not working as expected because you are using force:createRecord event to redirect to the Opportunity create page. When you use this event, you will always get redirected to the Standard create screen.

You will need to address the redirection based on the record type selected in your controller in createRecordFun JS funtion. You will need to use force:navigateToComponent (refer to documentation for implementation details) to be able to redirect to your custom component.

Below is a pusedo-code, which you should extend to get it working:

createRecordFun : function (component, event, helper) {
    var recordTypeSelected = <fetch the record type selected on the component>;

    if(recordTypeSelected === "A") {

        // use the force:createRecord event here
        var createRecordEvent = $A.get("e.force:createRecord");
        createRecordEvent.setParams({
            "entityApiName": "Opportunity",
            "recordTypeId":rtDet.id
        });
        createRecordEvent.fire();
    } else {

          // use force:navigateToComponent here
          var evt = $A.get("e.force:navigateToComponent");
          evt.setParams({
                         componentDef : "c:myComponent",
                         componentAttributes: {}
           });
           evt.fire();
    }
},