The Liber brevior simply states:

Just FYI, the Liber Brevior has the pre-1955 Holy Week (in which the cantors begin the litany while processing back from the baptismal font, but end up in the center of the sanctuary.)

The 1962 Missal gives both normal/solemn and 'simple' rubrics (18. and 18 a.).

18. says it is sung by two cantors in the sanctuary. 18 a. says it is sung by two cantors—without specifying a location—but if there are no cantors available, the priest himself kneels on the altar step and leads the litany.

So, cantors coming down from the loft is probably not necessary. They would probably miss much of the Kyrie on their way back after the 2nd part of the litany (assuming everything else is going smoothly/on time).

In the Easter Vigil the Litany of the Saints is sung in a very solemn manner. The rubrics in the Missale helpfully indicate that this solemnity should be realized by having two cantors sing the Litany while kneeling in the center of the sanctuary. In addition, the rubrics say that if cantors aren’t available, the celebrant should sing or recite the Litany from the bottom step of the altar. The rubrics thus indicate that the Litany should still be sung in a solemn manner even if cantors are unavailable.

Singing the Litany from the choir loft is less solemn than singing from the sanctuary. As such, singing from the choir loft is not what is indicated for the Easter Vigil. Now that said, in liturgical matters you are only required to do what you are capable of doing. So if there is a good reason why you can’t follow the rubrics, then you aren’t bound. But what you do in this particular case should be discussed and agreed with the responsible clergy.

Clergy attend the liturgy in choro if there are more clergy present than are required for the liturgy. This has nothing to do with the rubrics for singing the Litany.