[why is my echo '<script>alert("Cannot be deleted!")</script>'; is not working?]

Does <script>alert("Cannot be deleted!")</script> actually get echoed if you view the page source after the page loads?  If it is there, what doctype are you using?  I am no pro javascript guy by any stretch of the meaning but I believe that in most doctypes besides html5 you need to specifiy type="text/javascript" in the <script>, not sure if not having that will make it not work but worth a try.

[Keep checkbox checked after submit refresh]

I have a tutorial on this type of thing, it deals with the select box but the theory is the same, just change it to use the check box instead and use checked="checked"

http://amecms.com/article/Retaining-Drop-down-Selection-After-Form-Submit

[Array to Variable]

$title = $row['title'];

[Making mandatory fields in contact form]

As stated previously, form validation can be tricky and complicated. I have made a tutorial on this subject that should help you.

http://amecms.com/article/How-to-validate-a-form-the-right-way