I want to work with you on long term project. I've experience 2+ year in a software company on yii framework but i'm new on freelancer. I can give you extra work on weekend.
(1) Answer : I can use both themes basic or advanced, but i'll prefer basic app. If you want advanced app, no issue.
( 2 ) Answer : I'll use nullRef or fedemotta yii2 widget extensions but if you know better extension,no problem because you every extension has its own documentation.
( 3 ) I can work on both front-end / back-end because the company where i work, i've fully responsibility for my project.
( 4 ) I'll start alone on your project but if you need a team for your project , no problem we are two persons available in morning including me and overall we are four men and we are all yii2 professionals.
I hope we will work together and best of luck for your business.