Hello! These are all great questions. Some of them have to do with my priorities as both GM and Designer, so I hope the clarifications are useful.
1. This one is very open to how the GM wants to manage enemies and their numbers and perhaps their HP (if they have HP). How I tended to run enemies in my games is similar to how a PC works. A minion may be defeated with 1 attack (their luck running out), a strong foe might have the equivalent of 3 luck, and you could have a large villain have 10 luck, but of the players hit them 4 times they flee and now they're down to 6 luck permanently. OR you can make a player stat blocl for a foe and treat them similarly. OR you could add more crunch to the combat by giving them Hit Points and base damage off of a weapon or Thing, which is not something I define in the guide, though it is something I thought of as a potential option while planning test games. I tend to keep numbers small and favor hits in the mannor of Luck to keep things flowing and simple. If you want an example of a much expanded version of some of my mechanics; GURPS is very influential on me.
2. Defense is invisioned as your "Universal Defense" for any direct conflict. But conditions may apply. So if hypothetical character Elfara has a base defense of 10, Shining Armor +2 (which could be equipment or a Thing), is Dodgy +3, and has a Noble Brooch +1 she could have a max Defense of 16. But some of this is pretty conditional and it must simply match the tone of your game. In an Arthurian Legend you could argue her Armor and Brooch would protect her in almost any circumstance, be it at court or on the battlefield. Being Dodgy may help her avoid unwanted questions, it may help her tell a white lie. And it will definitely help her on the Battlefield. So she may have a defense of 13 sometimes or 15 other times. And to really answer your question: if that defense os penetrated in any way (physical or social wounds) then she loses Luck. On the Battlefield losing all luck may lead to death or capture, where as in court it could lead to enemies getting an advantage or her losing social standing with an important ally.
3. This is referring to Enemy TN, and therefore enemy Defense. "An Arch Rival may require a TN of 25" so that may be their permanent Defense score as a foe. And this could be lowered through a campaign due to PC action (if that is how you want to play it) or the PCs just need to grow in power to meet exceptional challenges. I could imagine finding a magic item, or a machine that cancels a bad guy's shield in a Star Trek scenario and that allows our heroes to finally strike the foe. But also it could be that the Dark Sorcerer or Darth Vader stand-in has a really high Defense forever and you gotta overcome it.
Thank you for the questions!