I was just introduced to the Munchkin game this summer and I got myself Munchkin Impossible and The Good, The Bad, and The Munchkin.

I have a few questions, due to confusion partially caused by the FAQ.

Q. If I'm mixing sets, are Steeds and Vehicles the same thing?
A. For purposes of mixing your Munchkins, yes. Steeds are Vehicles. Vehicles are Steeds. Cards that affect one, affect the other. However, Ships are neither one, even in a blended game.

1. Does Driver's Ed Training provide a +1 to Steeds?

2. Can a Cowboy with a Vehicle Equipped win the game by reaching level 9 (by killing a monster)?

3-5. Can a Vehicle be Smart? Even the Swiss Army Knife? Can Vehicles/ SAK carry the Saddle Bags?

Q. Can I discard my Steed to automatically Run Away like I can with a Hireling?
A. Not unless the card itself says you can.

This seems to contradict the rules, which say "If your Steed gives you a penalty to Run Away, you may discard the Steed before you roll to flee. You don't suffer the penalty, but the Steed card goes to the discard pile."

7. I bolded the word penalty, in this context is it referring to the Bad Stuff, or the minus x to Run Away?

The only Steed in the base TGTB&TM with such a penalty is the Jackass.
Smart says "It is so smart that it won't let you sacrifice it to escape a monster." Implying that it is an option to do so.

8. Is the Jackass the only Steed (I have) that can be sacrificed (or prevented from being sacrificed by being a Smart Ass), or does the FAQ over ride this?

9. If an Item card is normally two handed, but becomes Steam-Powered, does it get the +2 or +3 bonus from having Unlimited Ammo?

10. Can Item modifiers be switched from Item to Item on the owner's turn, outside of combat, or are they locked to the item they are originally placed on? (eg. Incendiary Ammo, Unlimited Ammo, My Grand Pappy's, Steam Powered, Silencer)

11. Does the inclusion of a flame Item in your equipment or the use of a fiery one-shot make your whole combat flame, or just that bonus? This is important for Monsters like Tumbleweed/ Loco Weed (fire attacks do x2) and Killer Jalepeno (immune to fire attacks)

12. The TGTB&TM cards that I have been considering Guns (for Munchkin Impossible compatibility) are: Belt Buckle Gun, Shotgun, Six-Gun, Seven-Gun, Gatling Gun, Derringer, and Blunderbuss. My friend tried to argue that the Civil War Cannon should count as one too. I pointed out the difference between missile weapons and Guns. Can someone here make the official call?

That's all I have for now. Thank you for creating such an addicting, easy to play (and teach) game with such great humour.

I was so disappointed when I saw that Munchkin Impossible expansion April Fools joke!

Oh, I just remembered another one.

13. What is the deal with Undead? Do they do anything other than the Cleric interaction? I've heard they work like the "In Black" cards, where they can freely join eachother, but I can't find any rule on it.

14. Are Traps and Curses always interchangeable, even if a card mentions one by name but not the other (Like Ambush)?

Re: Few Munchkin questions
 

It looks like Andrew might be doing something as he edited your post, but I'll give these a try anyway:

I can't say for sure not seeing the card, but the Vehicles are Steeds and Steeds are Vehicles description seems that it would make this clear.

Seems so.

Yes for 3. The other two I'm not sure, but use the Vehicles are Steeds and Steeds are Vehicles rule.

The minux X to Run Away.

It seems this might have originally been a mistake, or perhaps an oversight as the card seems based on Smart Pills from The Need for Steed. However, we do have that sacrifice on a penalty rule, so it would seem it would block that and MunchkinMan said the same thing here. Note that if there are any Steeds that provide a +1 to Run Away, then this can still be considered a penalty if your character has +2 or more to Run Away without the Steed, so it would stop you discarding that Steed in that situation too.

Sorry, can't find the effect of that card, so I'll have to leave this one for others to answer.

Item Enhancers are locked unless they say otherwise.

No it doesn't. Only the fire based bonus is doubled or zeroed. Note that the whole fire bonus is still fully doubled or zeroed if there are other monsters in the combat, though.

The Cheat With Both Hands rules say this:
Cannons are not firearms, so it wouldn't count.

This is a fairly new rule that is being added to new printings of each set. The errata pages for each set with Undead monsters show you the new rule. E.g., the errata page for The Good, the Bad, and the Munchkin say:
Yes they are. The CWBH rules (one of the best places to get info on Blending, BTW) say:

Few Munchkin questions
 

9. If an Item card is normally two handed, but becomes Steam-Powered, does it get the +2 or +3 bonus from having Unlimited Ammo?

Steam-Powered provides +2 combat to an item, and makes it take one less hand than normal.
...With Unlimited Ammo provides a +3 for items that require more than one hand (which the item did originally) or +2 otherwise.

Re: Few Munchkin questions
 

Since Steam Powered removes one Hand requirement, a 2 Hands Item becomes a Hand Item and gets +2 because of Unlimited Ammo.