1. Your account name does not need to be the same. When you created the Splinterlands account, it automatically created a Hive account, so yes it's you.

2. Now that you have two Hive account, you can log in as either one. It should not be problematic. But if you continue to post as supersamio2021 and receive SPT you will get SPS airdrop points to that account also. So remember to claim them.

3. You can always drop the @supersamio2021 account and use @supersamio for both Hive and Splinterlands. Just transfer any tokens to the one account and go.

I can't help with the Uphold account, never used it. But I can tell you that you cannot convert credits to DEC. There has been no announcement regarding PayPal supporting DEC at this time.