If you are very thin, it should be possible. In theory, 2-3 can be placed.

Will it feel like it doesn't stick to the back and feels empty when carried?

It's quite comfortable, the fabric is not hard, but it's very sturdy, with a very hard drag bottom inside.

Can I put down the A4 textbook?

I think the vast majority of textbooks can be put down, and 13 inch or 14 inch computers can also be installed.

What is the size of the 27 # bag?

The size is not too big & carries just what I need for conventions. The window is a great size for displaying pins. Different ways to carry around.

Is there inserts in this bag?

There are two inserts in this bag (top and bottom) that are removable for you to add your pins to.