Data, unlike agenda, remains the plural word that it is in Latin.
Unless firm data is available at an early date . .
This is wrong. Is should be are.
If a singular is wanted, it is usually one of the data, not datum.
The ordinary meaning of datum is:
Any position or element in relation to which others are determined: chiefly in the phrases: datum point, a point assumed or used as a basis of reckoning, adjustment or the like — datum line a horizontal line from which heights and depths of points are reckoned, as in a railroad plan. . . . (Webster.)