I'll try to answer each of your questions based on what I know and believe.

I might be wrong, but it's my opinion still.

a. Why Lakers, why the clandestine trade at night? They could have traded Luka to other teams and get a better deal?
You said it that the Mavericks are the one who initiated a trade, and the mindset of their GM is that "Defense wins games", and who's the perfect fit for that role? AD so they risked it all, traded their face of the franchise for a player that's averaging around 25 PPG and 11 RPG, but at the same time, a reliable paint defender. On the flip side, Luka is a liability when it comes to defense, and it doesn't align with what the GM has said hence, the trade happened.
b. The Mavs might have known something that they are willing to trade Luka, I do not buy the conditioning nor not playing defense issues. There could be more. And that reasoning? Is bullshit, you say that he doesn't have conditioning and that cause him to be injury prone like this season, but then got a player that is so fragile and coming from a abdominal strain injury.
Just for context, Davis is now way healthier than he is in his first years in when he went to LA. Just last season, he played more than 70+ games, and this year, he played in 42 of the first 47 games in LA. That shows that he's healthier now and wasn't as fragile as he is in the past, and this is a good improvement for him that made him a reliable paint defender.
As for the conditioning part, I don't see any reasons to disagree with it TBH. He's now weighing 270 pounds, a 40-pound increase when he was still rookie, and maybe for the management, they see Luka as the "NEXT ZION" hence, they traded him. He's a liability on defense TBH. He's like Harden back when he's on Brooklyn. He's one of the best in offense, but shit on defense.
c. Luka is their corner stone, a generational talent, and at 25 years old, is still very much in his prime as compare to Davis.
That's why it's a shocker for the NBA world because nobody expected that an MVP candidate like Luka will be traded at this young. Can't blame Mavs' GM though.
d. No one knows that Luka is available, nor he did request for a trade.
Both parties didn't expect that they got traded based on the reports. Luka, Davis and even Lebron don't know what happened until it happened. As for the availability of Luka, it's still unknown on when he will come back.
e. Mavs is the one who initiated the trade.
Yes, they initiated the trade because they're on a "win-mode" now. Davis is still injured, but in terms of health, Davis is the one that's healthier right now compared to Luka.
This trade as going to haunt the Mavs, and I think if Mark Cuban is has the power, this trade is not going to happen.
It will... only if they will not win this season because as much as I want AD to be on the Mavericks, they wasted a generational talent like Luka. I guess let's just see how this trade will go further this season. We also said at the Knicks-Wolves trade during the off-season that the Randle-KAT trade was just fair, but we are seeing who's the winner right now. Let's wait for this one as well. I just hope that the Lakers will somehow manage to sustain their 5th spot now that they don't have any reliable paint defender anymore.