Is it possible to buy Destiny 2 on Stadia (without Stadia Pro)
I believe so, but as long as a pro subscription is active I'm not sure you will have the option to buy it until it expires.
If you have Pro, you can't buy it because it is currently a claimable game for Pro users. But if you don't have Pro, yes you can buy it!
Yes you can, but if you want to get the "full" game there is a "update pack" with Renegat & Shadow keep.