Mfcttrf

At then end of "Indiana Jones and the Last Crusade", Donovan agrees to let Elsa choose which cup might be the true Holy Grail. As we find out, she (and by extension, Donovan) chooses "poorly." Is this an intentional tactic on her part to get Donovan out of the picture so Indy can select the correct cup without gun-wielding Donovan lurking over his shoulder?

If she's not trying to get Donovan out of the picture, what reasons does she have for selecting the false grail, other than it just being an honest mistake?

It was a deliberate attempt to take Donovan out of the picture so that she and Indy could take the grail. Elsa states during the movie that she has no particular loyalty to either the Nazis or to Donovan, she is only interested in the Grail itself.

Is that what you think of me? I believe in the Grail, not the Swastika.

She also shows distaste for Donovan's methods and motives, such as the look of horror she gives him when he sends his own men into the traps in front of the grail room. On the other hand, she clearly respects Indy, perhaps even loves him, and believes that they can take the real grail together.

It's ours, Indy. Yours and mine.

She clearly knew that the grail she handed Donovan was fake and would kill him as the grail knight said, as before she gives it to him the script specifies "Elsa and Indy exchange looks. He thinks he is seeing her in her true colors." and immediately after Donovan's death she notes that the real grail would not be made of gold.

Is this an intentional tactic on her part to get Donovan out of the picture so Indy can select the correct cup without gun-wielding Donovan lurking over his shoulder?

Yes.

Elsa clearly knows that the bejewelled gold cup is not the Grail.

In fact, she says so

 INDY studies the array of chalices.

 ELSA
 It would not be made out of gold.

Certainly she wants the Grail herself but is unwilling, at the last minute, to let it go against saving her own life. If you like that is her "poor choice".

It was likely the fact that she wanted the grail, so by choosing for donovan, she could test if it was the correct one. ( That's my opinion ). That way she had multiple chances to get the grail.