…on his scooter after he killed Freddie? In the movie he saw all the reporters in front of his building, so it gave him a way out of taking her there. (He dropped her off nearby, then went back to his apartment on his own.) But what if the reporters hadn’t been there? Why would he have set himself up that way? Or did he already know the reporters were there?
Likewise, soon after, in his apartment, Tom (as Dickie) tells the police investigator to let Marge in, then changes his mind a beat later. Calculated? Or did he honestly slip and forget he was Dickie for a moment? (Hard to believe the latter.)
In both of your examples he is doing it to appear more credible
If the police hadn't been at Dickie's apartment we can presume that Ripley would have either murdered Marge or found some other excuse to not let her up into the apartment
I agree with the first ... that he brought Marge to Dickie's building to seem credible.
Writer & director Minghella's commentary explains that this scene illustrates Ripley's ability to improvise, like a jazz player playing variations on a theme.
So Ripley didn't really know what he'd have done had they not run into the police (the reporters were there when he returned). He was, after all, already rattled from crashing his scooter a moment earlier. He was improvising.
And I WOULD agree that, in the second instance, it was to seem credible; i.e., he pretended he was OK letting her in, then pretended to change his mind.
However, Minghella explains that Ripley, at this point, is "spinning plates" frantically, and had actually forgotten that Marge was on her way. When he said to let her in, that was Ripley legitimately giving up. He was weary at the world closing in on him.
But then his instincts kick in and he starts improvising again.
It's ridiculous, but this thread actually conjured up an image in my head of Tom sticking his head around a wall and pretending to speak as Dickie.
Specifically called to mind a scene from the Gene Wilder comedy, "The World's Greatest Lover" at 34:30
Matt Damon's acting, or the direction of it, is so good in this scene that I interpreted it the way you said Minghella describes it.
"Tom" is no longer acting when he says "let her in, what's the difference", it genuinely looked like he had given up and was resigned to the fact that Tom/Dickie was going to get sent down anyway. Then it's like he gets a second wind and starts acting again, like he says, instincts kicking in because there's still a chance, however slim, that he can get away with it, but that would all be gone if Marge entered the room.
As for the first question of the OP, I'd assume he was going to kill her in the empty apartment but had to change tac when there were people outside. That said, Marge also disappearing would have been too much....although maybe he'd already made the decision, even at this point, to write Dickie's suicide note and go back to being Tom. So one more murder attributed to Dickie wont make a difference.
I think it was simply a bluff, put yourself in the shoes of Marge, Margie was already starting to have doubts about Tom, how he came into their life and now Dickie is missing.
If when i see you and you ask me where Dickie is, i start to stutter and say "mmm mmm he's, heee's mmmm he's just gone out to eat..." you're going to sense that something is wrong.
If very confidently i tell you "Dickie? let's go i'll take you to see him right now..." Then Dickie is very much alive as far as i'm concerned and i haven't killed him and i can even take you to him. that level of certanity and "let's go" had Marge really taken aback.
It's a bluff, that leaves it seeming that Tom could take her to Dickie, But Dickie didn't want to see her. total bluff.