Not sure about the included membership, but PS+ Physical cards are codes to redeem in the PS Store. You may also purchase it via credit card on his account directly to have it applied to that account.
One: You can back up save files and then restore them at a later time even to another account. If you are a PS+ member you can back them up to a server and they will be linked to your PSN. However in your case, you'll probably want to back them up manually, which can be done as so:
- On the XMB (cross media bar), go to the Game section.
- Go to Game Saves
- Put in your memory stick (cf, ms duo, sd)
- Find the game save you want to copy/backup, highlight it and hit the Triangle button.
- Choose the menu option to Copy
- Hit the X button on the memory stick and let it copy.
Source: PS3 : How to backup PS3 game save files to memory stick / sd / cf
It should be noted that some games use copy-prevention methods to, well, prevent copying. However, it's still possible to back even those games up. Google is your friend here.
Two: I'm surprised the previous owner's PSN is still on there, but yes you could use his DLC, as long as his PSN remains registered on the console (so don't delete his User account.) Off the top of my head I don't know how this will affect trophies. You should still be able to earn DLC trophies, but they might register for the owner of the original purchase. (The PS Vita has this problem.)
Three: The PS3 is not region locked for games, so you should be able to play everything fine. However, I do believe the Playstation Store is region based, and so some content will only be available depending on which region's store you access. I'm not sure if this is determined by the console (e.g. a US PS3 vs. a JP PS3) or registered PSN region. Also, you may have some conflict if trying to purchase content that is normally not available in your region.
I hope this helps!
P.S. Congrats on the new PS3!
Best Answer
No. If you activated both PlayStation Plus memberships separately for you then your brother then the subscriptions will be concurrent on the system.
You would have had to have activated both cards on the same account for them to add on to one another.
As for the second part of your question. Normally the only time there is complementary time is when Sony runs trial promotions, or sometimes they bundle free time in with systems. If you add a purchased time to an account created with trial time the paid time is tacked on to the trial period. Eg, you sign up with a 30-day trial then 1 day into this you pay for a 3 month card and enter the code. You'll have 4 mos of PSN+ before it terminates.