While I find Trixie being God a bit outlandish, I do like the theory that god is trying not to test but to teach Lucifer. My guess, drawing a bit on the original Vertigo comics, is that for whatever reason God is starting to withdraw. It’s been stated in the past that most of the angles don’t even know what his intention is, hence Uriel’s fuckery. Maybe he just wants to retire, maybe he’s dying like in the comics, or maybe he’s just getting weaker. All of these might explain why the Goddess was able to escape, and how her power was coming back, if the the entity responsible for suppressing her was weakening.
It’s been stated repeatedly that Lucifer (pre-fall) and Amenadiel were his favored sons, Lucifer’s name Samael being a sign of his favored position (I do feel compelled to point out unrelated that in reality Lucifer, not Samael, means Light Bringer, Samael meaning Venom of God), and Amenadiel’s position being shown by his necklace. But both are nonetheless deeply flawed characters when we first meet them, not by any account ready to inherit a universe.
Amenadiel is rigid and uncompromising, lacking compassion. But by his fall and compulsion to live among humans, he has developed compassion and affection. He is no longer merely the wrath of god, he is learning to care for humans, truly, they way god does, and he is learning something god didn’t learn until it was to late, his own fallibility.
Lucifer, meanwhile, is hedonistic and uncaring. Through Chloe god is helping Lucifer learn the values of attachments, friendship and love, and through Chloe’s interactions with him of forgiveness. I suspect, as evidenced by the return of his wings and everything that happened last season, god has already forgiven lucifer for his rebellion, and even regrets his own reaction, but he knows Lucifer won’t accept his apology, won’t believe its sincerity. God has come to grips with his fallibility, and is teaching Lucifer not to make the same mistakes he did. In many ways Lucifer is the most like god of all his siblings (That we’ve seen), something Linda hints at on occasion. Lucifer is manipulative, impulsive, prone to blind rage, vindictive, loves humans, and yet is emotionally distant. The first season taught Lucifer the costs of his impulsiveness and set up his friendship with Chloe. The second season helpe him recognize the costs of his vindictiveness and grow past it. Now in the third season his distance has cost him Chloe, and he’s learning its costs. Each of these events helps Lucifer grow into a better person, an better ruler, than his father.
In the end, this is just my opinion, but I think it’s well supported in the show and source material, and I think it is an interesting path for the show to take.