Well, we wouldn't be having this conversation, for one thing.
A basic principle when building any UI is that when a user changes something, s/he should immediately see some result of that change. If someone deletes a submission and nothing looks different, that can get confusing. Case in point.
There's two confirmation boxes. The first asks if you really want to delete the submission; clicking Cancel here will cancel the whole thing. If you click OK, it will then ask if you want to send a PM. Clicking Cancel here will not send a PM but still delete the submission; clicking OK will prompt for a message, send the PM, and then delete the submission.
I have looked at the code and there is no way for what you described to happen.
AdBlock Plus: Ctrl+Shift+V to open blockable items.
Search for "embed." Double-click the one item that remains.
To block all embedded Youtube videos everywhere, check the second radio ("youtube.com/embed/*") and click "Add Filter." To just block embedded videos on KC:MM, check the same radio, click "Advanced View," and check "Restrict to domain."
You can tweak the filter as you see fit.
Edit: Actually I got tired of this about five minutes after I did it.
I gathered that access from different devices could result in the ban of an account (or IP?)
I don't know where you got this impression. It's certainly not in the rules.
Kit has a separate "cell phone" account because it has different settings (theme, etc.) that make viewing the forum easier on a mobile device. She made and named it of her own accord; we didn't tell her to do anything.