In my game, Ashley did it, and she was the one that held Shep's nameplate during the "funeral" scene. I'm assuming it's because she was the LI back in ME1 while Miranda was in ME2&3, so the game picked Ash. Garrus would have made more sense, but I imagine he was too overcome with grief and had probably been drinking to numb the pain.
Was Ash in your squad in the dash for the beam? If so then I have no idea what the criteria might be. As I said, in my game, Liara was the LI and I had her in my squad during the evac scene and it was Garrus that had the scene with Joker.
It wouldn't make much sense for either of your squadmates to be in that scene since they should have been down in medbay at that point.
^There's a difference between seeing fellow solders die and watching a child you spoke to just 10 minutes ago get blown to tiny bits. Aside from being something of a riff on 'Schindler's List' (remember the girl in the red coat?) the kid is supposed to have come to subconsciously represent all those Shepard couldn't save.
As for why the Catalyst took that form, it's a fair bet that it wasn't a hologram, but was a direct communication with Shepard's mind. While I don't buy the Indoctrination Theory, it's a fair bet that the Reapers got inside her skull through TIM and chose that form from her subconscious.
Oh, I understand what the kid is supposed to represent. I just think if they were serious about having your choices "matter", then tying that ghost kid to an actual loss from the very first game would have made much more sense.
The problem with that idea is that it's not universal for all Shepards. Aside from practical concerns like it doubling the memory budget by having to record the dialogue twice, what if you're playing a full on renegade that doesn't feel at all bad about the loss of Kaiden (I know I didn't!) It also risks alienating anyone who hasn't played the previous games as they'll have no clue who that person is, nor why they should care. Plus of course this would place that particular character's death as "important" over and above anyone else who may have died since in ME2 & earlier in ME3, which again is not universal to all Shepards.
They spent a lot of time in the game establishing the recurring dream and what it means to Shepard. The kid is just a symbol. Indeed, after one of the dreams (the second one I think?) Shepard can tell Liara that she was thinking about whomever died on Virmire.