I'm not the one who brought it up. And it wasn't enough of a discussion to prompt multiple paragraphs reiterating stuff we already know.
None of these handwaves are in the game itself and some of them only prompt more questions. If Merlin and the Fairy Godmother used magic to restore Radiant Garden why didn't they do so before? In the year between KH1 and KH2 the place goes from a ruin with only the castle above water to a place that was recovering, but still a long way off from its former glory. In what was at most a few months but was possibly as little as a few weeks the city went from recovering to restored as if the previous decade had never happened. There's actually a big difference between the two pictures you provided to the point that I suppose "magic" really is the only way you could achieve the result in such a short time period.
Really, if they didn't want to come up with a new design for post-KH2 RG then they should have simply not shown the world outside the castle. It's not like the two scenes that take place in the fountain square couldn't have taken place elsewhere. It was cool to see the world in UE4 graphics, but not worth the disrespect to the world's history.
Time moves differently in all worlds. Problem solved.
I mean, really? Is it that hard to just buy that Hollow Bastion from KH1 gets fixed to what we see in KH3 over the course of several games? But also THAT is the biggest disservice to Hollow Bastion? An appearance change? To be brutally honest, I actually think that your line of questioning should be applied to Hollow Bastion in KH1 to what we get in KH2, but of course no one bats an eye at what KH2 does. It's established that Hollow Bastion was literally destroyed by Maleficent, NOT taken by darkness, which means that after the events of KH1, it wasn't going to be magically recompleted like other worlds. Leon and the the gang physically left Hollow Bastion on their own, NOT by being displaced by the darkness consuming the world.
So for Hollow Bastion to be utterly obliterated and nothing more than the broken down castle in KH1 to turn into the town and everything mostly fixed in KH2 is WAY more shocking and questionable than how the world was in KH2 and what it turned into in KH3. I didn't question it, but since you're bringing it up I want to know why you don't think that's as questionable as the discrepancy between KH2 and KH3. I can buy time and magic fixing up Hollow Bastion into whatever state Nomura wants it to be in, but I'm curious why you didn't question Hollow Bastion's drastic appearance change from KH1 to KH2.
Aside from that, what I think they actually did wrong with Radiant Garden in KH3 was not using the Final Fantasy characters in conjunction with the ex-Organization members at the castle. Not having them interact with Ansem The Wise (or maybe even be part of the storyline to help him). Never having the Final Fantasy characters want to hunt down Maleficent. Not using the world as an actual hub world or even a mid-point world.
And on top of everything else... its cutscenes occur on the Gummiphone. I rather have Sora complete the DISNEY world, then the cutscene just opens with Sora and the gang in Radiant Garden chatting about whatever Ienzo had to say. They could have even had the conversations be in different locations - the lab, outside the castle, the garden, the fountain court, by the Villains' Vale (and maybe even explain what that is or say someone is investigating) etc. I would have even been fine with the world as just explorable parts where we hear segments of the story being told as we traverse to where we need to go. Like Sora lands in the garden, then the mission info is go to the castle and talk to Ienzo. Along the way, we hear Sora/Donald/Goofy having the conversation they would have had in the Gummiship and hear their thoughts about things, maybe even have people along the way you can interact with like Leon. Then get to the cutscene with Ienzo. Kind of like how they do it in The Last of Us where there are scenes with no action, but just chatting as you get to the next story location.