Multi delete isn't an official feature of DA. We would probably create a new extension to handle that scenario rather than document how to hand code to get it done, it is just that currently that is the only option.
You are going to have to work sequentially. It sounds like you are trying to delete multiple files based on a checkbox.
Start by getting it to delete a single file.
Then get it to delete the single file based on the checkbox.
Once those are working then try to get it working for multiple files. That will help you identify exactly when and where you go wrong. You may also want to learn about using die() in your code to debug. It is a valuable on-line debugging tool that will help anyone trying to write code by hand and understand what is going on.