never try this website yet. 
most site I tried get their gift/xbl gold from shady grey market sources anyway 
cdkeys, nokeys, g2a. to me as long as they have a paypal system Im cool. can just contact paypal & request to refund my order.
imagine get 12 + 1 months XBL Gold cheaper then PS+ MY RM129 
if you want secure site try buy from Amazon. they do some time make sale like buy XBLG 3 months for 15 pounds get 10 pounds gift card for free

Finally buy from cdkey currently the cheaper after compare.. USD45 after discount code for 1 year account

1. Xbox One games are pretty much the same wherever you buy/redeem from, since there are no longer region restrictions like the old days. As long as your Xbox is set to the language you want to use, it will download the local language of the game.

2. That's not accurate. I don't have any issue using my credit card to buy digital games directly from Xbox Live. I've bought using Citibank and Maybank with no issues. I can't be sure about other banks though.

3. All your purchases should carry over when you change region. I haven't tried it recently, but the last time when I changed my region from Singapore to US, all my games are still in the account. BUT, any credits you have in the account will not carry over to the new region.