Just saw the video! It was awesome! I like your style of let's play video! Congratulations btw for the Lamia, she is the hardest boss by far.
The demo is almost finished. After the dungeon you're currently in (Mount Grevis), which is shorter than the previous dungeon, you'll have one boss fight (which is easier than the Lamia) and then it's done! One more video should be plenty enough to cover the rest of the demo.
To answer some of your questions:
1) Elemental resistances: the percentage indicated is the percentage of damage the victim will receive. 100% means no resistances and no weaknesses, 80% means it will take 20% less damage (so less than 100 means resistance), and 120% means it will take 20% more damage (so more than 100 means weakness).
2) Abilities: You had a good grasp of the system at the end of your second let's play. All the effect listed on a skill are active. Using a sacred tablet (or emblem) will simply level up an ability, making its effect stronger. That way, the player can customize his skills to his liking and choose his builds. For instance, for all of Francis' spells, you can decide to build the skill for single target damage or focus on AoE damage.
3) Hero Abilities: Hero abilities are different than skill abilities (the system you've been using in the second video). Hero abilities are general abilities that apply to all the actions of a hero. Like the skill abilities, all the hero abilities listed are active, and using an emblem will level up an ability an increase its effect. You access hero abilities through Status -> Abilities.
4) Magical items: Those that are listed in the window on the right when you inspect your skill abilities (the item highlighted in blue is the one you need to upgrade the ability your cursor is on). All the "sacred tablets" are used to level up skill abilities, and all "emblems" are used to level up hero abilities (I think you have a couple of them in your inventory).
I also have a question for you: do you think it would be better to simply remove the action sequence mechanic (during the regular attacks) and simply make it so the player receives his 3 souls automatically after the attack? Some people said that they didn't like the action sequence thing.
As for your speculations, they are mostly correct lol (I guess a jrpg isn't that hard to guess haha). Azel and Marianne are the main protagonists, so yeah... relation between them? Maybe??? (spoiler alert). Don't worry about Francis, he's older than the others so he won't steal anyone's girl. As for the relation between Azel and Yukari, they are like brother and sister since they grew up together in the same home (that part is covered at the beginning of the complete game), so no romantic relationship there haha (unless people are into that???).