The short answer: start over and do it right.
Yes! I agree!

Guessing aside, and taking a clue from this "diagonally" expression, I
would offer that you look at the antenna from above as you attempted
in your picture and see if it looks like:


+
|
| +
| |
| | +
| | |
| | | +
| | | |
.---+-----+-----+-----+----. two rails, one hidden by top
| | | |
| | | +
| | |
| | +
| |
| +
|
+

And confirm that each element, in order on one side, alternates
between the two rails that hold them.

This is what I think, but how can I explain to our managers, who got
the lab set up by a contractor, that they (the 'experts') assembled
the antenna wrongly? what are the possible effects on the measurements
to be made?

As you may have seen in the pdf, the antenna consists of four beams,
in two pairs, meeting at the front (feed point?)

All the longest dipoles are on the right-hand side!