For requirement#1 you need to use this extension point under "ViewExtensions" part of your component.js file. The name of the extension point needs to be defined correctly or else it will not work.
Here is a sample:
"CustomerExtensionForInfoTabContent": {
className: "sap.ui.core.Fragment",
fragmentName: "zxyz.approve.frag.S3_CustomerExtensionForInfoTabContent",
type: "XML"
}
Regarding other requirements #2 to show Notes and Attachments tab, this is available out of the box for the app. you do not need to do any customizing or extension for this as if your backend SAP Document has Notes and Attachments in the WORKFLOW CONTAINER then it will show up in the app. Please check workflow log and the container and see what is coming in the attachment and notes container.
I have all of this working for Notes & Attachment without single line of code for My Extended Approve Requests Fiori App. I have also added custom fields in the Info tab view by using above extension point and it works fine.
Regarding your point of Kony v/s SAP Fiori please check all options before making decision. Fiori is based on open industry protocols like oData, HTML5, JS and you are not tied to any proprietary software. Fiori is very extensible and easy to use. it is your decision finally but consider everything before you make that final choice.
regards
Ashish