Hello,
I am experienced web developer working with Yii.
Below are answers to your questions, and after you send me more details/mockups we can discuss project further.
All answers are based upon my knowledge. I am backend + frontend developer. For design I do not do it, but you said you have it already
1. How scalable is that framework?
-If will work in Yii it is very scalable
2. How is it to adapt to future changes? ( Even after complete development, I will add my creative ideas or any new features.)
It is very easy
3. How does it appeal to SEO?
It has rewrite module, but naturally SEO is empty. For very controller you have to define appropriate metatags. For such big projects it is not the problem. On the other hand it is very good, because google likes professional, speed sites. And most of them are.
4. Whats the ease of use?
For experienced developer very easy. For others maybe hard to understand in beginning. But websites/apps make on Yii are easy or hard depending how good the developer did the job and how it is described before.
5. How long will it take to complete the development? (in hours, & in days, & weeks, & months all separately)
I do not know, you have to send more details/screens.
6. How much would this cost approximately?
Same as above. For long projects my price is 20$ per hour, it can take 2 or 3 months
Thanks, Zivorad