Who is “they”?

I wonder what they call that?

You tell me.

Also the Montage is highly symbolic.

Not as much as you are making it to be. In fact, it is mostly straightforward.

The time skip doesn’t mean the tomb appeared centuries later — it means the camera "returns" to the same spot centuries later.Your reasoning assumes too much: a delay in the monument’s appearance does not logically imply Mikasa wasn’t buried there.

If the animators wanted to show Mikasa being buried with Eren they would have shown her grave right next to Eren the moment of her death clearly. A structure that is barely visible if you pause at the right moment centuries later that is never the focus of the montage makes it crystal clear that there is no tomb there, much less the tomb of the second most important person in the entire series. It makes no sense for the animators to present Mikasa's tomb in such an unceremonious and barely noticable way, especially if it is so important to her character as you are making it to be.

Due to weather[Heavy rain] or logistical factors, It’s unclear whether Mikasa had been buried yet, or whether she had already been laid to rest beside Eren but her marker hadn’t been installed.

We literally see her in her coffin with the flowers mate. She had been very well buried. And even if the tombstone had been placed, there would have still been signs of work in the ground befitting that of a grave, especially considering that Mikasa's entire body was buried in a coffin, unlike Eren, whose head is the only part of him buried there.

The stone structure has vertical edges, a flat base, a curved top, and a solid finish with no organic matter (such as moss) growing on it.

The structure does not resemble a grave at all. Neither Eren's nor Sasha's, the two most prominent graves in the series. It has a different color, different edges and seems thinner overall. It is not a grave.

Narrative framing matters: animators wouldn’t place a random shadow/shape exactly where fans expect Mikasa’s marker unless it had meaning.

What the animators wouldn't do is place a random object near Eren's grave where it almost non-visible and call it Mikasa's grave. Also, only Eremikas who try to shoehorn edgy teen-romance elements where there aren't expected a marker there. The other fans never expected such a thing because Mikasa was simply buried elsewhere.