Please excuse my silence, as I've got my foot in my mouth. I have just re read your first post.
1 Remove the account you added to Gmail.
2 Add both accounts to Windows Live Mail.
Now for the tricky bit. In your Gmail, POP & IMAP are both enabled. How do you access your mail, home PC only or other. (As both are Web based, they can be accessed on any PC by usual method) In WLM, I prefer accessing Gmail using POP, so if IMAP is not required, disable it.
I have Gmail set to delete mail when accessed using POP, (their are other options) but can still access it in Gmail if needed.
Now to your Windows mail account. I don't use mine in WLM except to sign in, then I get auto access to my contacts, and 'Safe + Blocked' sender features. If windows mail messages are deleted from WLM, I don't know if they remain as Google does or not.
Hope this makes sense.