I am new to Steam and this is only my 2nd HB experience. I have a few questions please.
1. If I create an account at HB does that secure my download page links better than if I didn't have an account? Or should I copy and paste the keys and save them on my computer and then delete the link page at HB?
Yes, though signing up for an account at HumbleBundle will also allow you to reclaim any bundles purchased with the e-mail used to create the account. All of my previous bundles were reclaimed when I finally got an account.
2. Is it best to enter the keys in Steam or Origin ASAP?
Depends on what client you prefer to launch from. It's probably best to activate through Origin if you don't have an aversion to it or already plan on activating some Origin-only games. They do have live chat for tech support on Origin, which can be nicer. No matter what, you will still be getting all the EA DRM on these games, regardless of using Steam or Origin.
I'm no fan of Origin, but I had to install if for Crysis 3 a while back. It's not as bad as it used to be. EA is still the devil, but they aren't taking a dime of the money given to HB for this bundle.
3. What if I already have a game or don't want one of the games? Can the key be traded or given away? What if I already entered the key in Steam or Origin?
Morally, and legally, you aren't supposed to split up the codes. Technically, nothing is stopping you from splitting up the codes. Unlike the Deep Silver Bundle that only had three codes that packaged multiple games together, this bundle has all separate keys. It seems most (I haven't checked all of them yet) of the games have DIFFERENT keys for Steam and Origin versions. The only oddball is the Sims 3 Date Night DLC which is redeemed through the Sims web site.
4. What is the deal with the soundtracks? Are they for the games or just for listening? Or do I download them and add them to Steam or Origin somehow?
They are for your listening pleasure. Humble Bundle started the tradition of offering something "extra" for gamers in these bundles. It might be prototype versions of the games, soundtracks, or even source code.
For those with a full hate-on for anything related to Origin, step back and re-examine things. I resisted Steam when it first came online, and while EA will never really let Origin be anything but a wannabe "Steam clone" marketplace for EA's offerings, they are working on it. There are a few worthwhile Origin-only games. Sales talk, and EA has already backed away from many stupid things they've tried to do, but missing out on a game you paid a few bucks for (none of which went to EA) just to avoid Origin is kind of silly.
If it helps you see things more objectively, consider that there are people who hate Steam as much as some of people hate Origin, and they aren't playing ANY of the games in this bundle.