1) The amount of you lag you'll deal with is the same no matter what, so technically speaking it's most advantageous in terms of followups to be as close to the ground as you can to get ready for a follow up ASAP. The basic principle of it is: If you short hop it instead, unless you time it right (which would be slower than just DJC) then you suffer the immobility in the air while you're falling (the distance between you and the ground when you start it) AND the cooldown of landing with PKF (Which, admittedly is very low) Since it's faster to do a DJC one, you should always go with it when you can if the purpose is to apply pressure/start to get closer.
It also allows you to combo into DACUS depending on how far away you are~
2) Pretty sure there's like 1 Frame to do this in, while it would probably be good to learn it, it's probably not worth it so early on in the character's lifetime to worry about it since the distance boost isn't THAT large
3) It's always a good idea to mix it up!
4) This is the thing where you fly in a direction of PKF, right? If you want to be flashy/have the positioning for it I suppose you COULD, but I think it's more for movement/being cool looking than anything. This'll probably be more of a player by player technique.
5) Sounds right. At least on the ground, it feels like you can jump sooner after an aerial Magnet, I'd like if a different Lucas could chime in here.
6) DACUS is a very powerful technique, so if you get good mileage out of it then that's definitely good. However, never settle into tunnel visioning into a solitary kill move. It's important you keep track of the options you have because there are plenty of other ways to kill with Lucas.
7) Nair isn't the BEST approach option you have, to be honest. PKF is definitely good since the distance you're at when you do it makes it fairly safe and Lucas is quick enough to get good confirms off of it. Other than PKF, a well spaced DJC Fair is also very good but you should always mix-up your approaches so that you don't get baited/read.
8) With all the options Lucas has, you should never be really reliant on anything
However, if you CAN get a grab it's important you note player tendencies and punish them with a grab when you can since Lucas has a great grab and throws. At the very least, it would be a good idea to switch between a jump-cancelled grab and dash grab, since they are fairly different in terms of when you do use them. If you have faith in getting that grab, then go for broke!
9) Uair is stupid good and has a very large hitbox, so you have to the potential to stuff people as they're falling. It's great in juggles since, even with DI, it's pretty simple to follow up and branches out into a variety of different moves. You sure won't be killing with it, I can tell you that much. It does link into itself pretty well.
10) Invincibility is one of the best parts, outside of the insane startup and full-body hitbox. The risks, while there, come with pretty good rewards. This is one of the hardest moves to integrate into your regular play, but it does have its uses. One of the easier to grasp ideas is to use it OOS to end pressure from the opponent, this IS MU specific though and the move's best uses will take time to develop.
11) I believe they did change it so you can't do actions right out of absorb. This happened back in 2.6 if I'm not mistaken.
And a casual question:
12) I don't think so, haha. I've gotten into the habit of calling USmash PK Love, as Kally brilliantly suggested (it even comes from his chest!) Otherwise we just stick to the regular stuff. Even though PK Fire is also PKF we still refer to PK Freeze as PKF, so that should give you an idea of what we use, haha.