Just bought one at g2a using discount code GAE (5%) for $32.34 including payment fee. Got the key in under 2 minutes.
https://www.g2a.com/xbox-live-12-months-gold-us-subscription-card.html
Edit: For anyone wondering about the regions, it says it's US only but I activated it in the UAE without issue.
Edit 2: A mate has just used it in the UK with no issues.
You can redeem the Xbox One games as long as you have Gold status.
You keep all the 360 titles, but the Xbox One titles you will lose if you lose Gold status. You can either redeem your Xbox One games on the system, or you can log in to the website and redeem them online so they're bound to your account:
https://www.xbox.com/en-US/live/gold
Always remember to check every on the 1st and 16th every month for new stuff - it's the golden rule
Yes the things you said.
The base version still exists on the store, but is only available for download if you purchase the definitive addition. e.g. with https://www.microsoft.com/en-gb/p/shadow-of-the-tomb-raider/brvk4w7rvcxr?activetab=pivot:overviewtab
There is however a way around this nonsense.
Buy the disc from Amazon or an alternative retailer.
The UK version is £14.97 currently - a significant saving, you may find a similar price on in your country (I'm assuming you're in America?)
This is generally the case with a lot of games, it's a lot cheaper to buy the disc.... The reason is to do with publishers - console manufacturers do not set these prices, hence the pricing does not always make sense.
There are also websites that sell the codes to redeem a downloadable version of the game, I prefer this as I find it more convenient to have a digital library - you may be able to find one, these can be cheaper than the official store.