Can I delete this account?

Yes you can get your old user name back, as you know in your profile it will not let you change your name because it keeps saying that your current email is already in use with the name LAPIII.
So what you have to do is go into your profile and you will see a option that says you can change your email address, so change your email address by just making something up like > Detfor@bob8875.yahoo.co.uk, then press submit,
You will then get a message saying that a email as been sent from toms hardware to this email Detfor@bob8875.yahoo.co.uk to confirm your account,
This will now free your own email up from Toms hardware so that you can rejoin by making a new account with the User name Advice Pro.

HOPE THIS HELPS.

This is LAPIII and my e-mail address is different than the one under Advice Pro. Why does the "Profile Header", to the left of each post if I click on More Information, still read stranger:

because you have only made 7 posts using the name Advice Pro, as time goes by and you put more posts on Toms Hardware, the name will change from stranger to something like enthusiast / addict or old hand, these names will only appear when you have done over 100 posts and higher
The same happened to me when I changed my user name and started again, the Forum sees you as a new User so you have to start off as a stranger because you have not done many posts,
just look at other peoples information and you will see that there names correspond with the amount of posts they have done.