http://wallet.google.com/

Remove your existing CC and re-add it using your card details but US address instead (I obtained US address via HopShopGo with my PayPal account).

And somehow my Play Store (accessed via Web without VPN or anything) became US' version of Play Store

I don't even know which is the reason that caused it as I've tried various methods just now.. I guess it might be after I changed my Google Account Country to United States.

Yup, can access all those but haven't try to purchase them yet (not planning to at current stage )

1) https://www.google.com/settings/account
2) There should be a header Email with sub-headers Primary Email & Secondary Email on the right. Look for the 'EDIT' text right beside the Email main header.
3) You'll be lead to https://accounts.google.com/b/0/EditUserInfo?hl=en_GB and thus, modify your Country (optional) there. Not sure if it'll work or not as initially it doesn't work for me. It just somehow happened to switch to US store after I finally gave up