As per my understanding for your first question
You can't change or you can't include your custom code in already existing minifiled.js(bundle.js) file to develop the functionality.You need to include one more JS file and extend the minified.js properties and functionalities in new JS file.
for second question minifiedfile.js file is nothing but bundle.js in my application,whatever the code i have shared with you.
For fourth question, If you want property panel at the bottom then you need to change in CSS.
If i am wrong with this answers anybody please suggest me.