As mentioned above, one possible consequence in DA2 would have left Merrill's mirror destroyed, so it could not have appeared during Inquisition, but there is no excuse for Varric not having at least mentioned it to the Inquisitor in an offhand manner.
Bodahn and Sandal's absence from Inquisition is probably the most ridiculous gaffe made by the devs, and they know it, because they sure as hell have gone out of their way not to offer a proper explanation. That diary found in the Fade during Trespasser is at best merely a concession to repeated comments about Sandal made here in the forum since the games release!
The mirror you see in the crossroads is the counterpart to Merril's mirror. So even if her mirror was destroyed, its counterpart is still intact in the crossroads.
As for Sandal, I dont think it was a mistake. They added too much hype to Sandal with his prophecy in DA2 to "forget" about him. I just think Sandal isnt present because whatever the revelation behind him is, we are not yet in a point in the story where it would make sense to learn about. Im sure he will show up again in the future, when it is proper for us to learn about him. Also, besides the diary in the crossroads, there is a dwarf in the dreaming sequence of the Templars quest (where you go to Therinfal Redoubt) that looks strickingly like Sandal caged in a cell. Both references to Sandal seem to indicate he is around, in the fade/crossroads, which is by itself a big revelation with him being a dwarf and all.
Also I wouldnt be surprised if later on we find out that the dates in his diary (in the crossroads) have some meaning that we can't understand now.
I admit, I was disappointed we didnt see them. But I dont think it was an oversight. We just have to wait and see.
EDIT: Now that i think about it, Sandal in a cage in a dream sequence, could mean that during Inquisition's timeline Sandal was a prisioner of the Qunari? In Trespasser we see that the Qunari were gathering magical artifacts of any and all kinds in Darvaarad, maybe they captured Sandal as well? And somehow Sandal escaped before we got to Darvaarad thru the eluvians. Remember the impaled Qunari next to Sandal's diary? Dunno, its all speculation, but it kinda makes sense. Maybe some DA lore scholar could look at the Diary's dates and try to make some more sense out of this?