To the first question, if you compare the new Graduale, you will find additional verses for the Introit, but they all use the standard mode VI tone. The traditional Introit uses a unique tone, which I have found very difficult to adapt to these other verses successfully. I imagine, given all that usually precedes a traditional Requiem Mass, and what is dropped at the altar, any more verses were never deemed necessary. When singing a traditional Requiem, I have always found the one verse and (interminable) Kyrie more than enough.

As to the second question, you have to look at the overall shape of the piece. "Dicite Dominus Implete" is clearly mode VI through the third line, what with all the te's and recitations on la. The rest is much more mode V, reciting on do, with the characteristic use of ti in the last line. Because this mode V feeling dominates the end of the piece, the transition to the verse works better if they are in mode V.

The same sort of argument could be made for "Mirabantur", with its recitation mostly on do, rather than re.