There are two distinct questions here: why "sein" and why Konjunktiv.
This is part of a fixed phrase: "zu sehen sein", which means "to be visible". In contrast, "sehen können" means "to be able to see". This is why "konnte" would change the meaning and would change it to nonsense (setting aside that "voraussichtlich" wouldn't work well with the Präteritum "konnte"): your suggested alternative would mean:
Wegen [...] konnte der Merkur voraussichtlich [...] vorübergehend sehen.
Because of [...] Mercury expectedly was able to see for some time [...].
Most planets, to my knowledge, can't see - not even expectedly and only for some time. ;-) Whereas with sein:
Wegen [...] ist/war der Merkur voraussichtlich [...] vorübergehend zu sehen.
Because of [...] Mercury is/was expected to be visible for some time [...].
The part being in Konjunktiv is reported speech. The part before:
"Östlich von Emsland ...", sagte ein DWD-Meteorologe.
is direkt speech and what goes between quotes are the exact words he (the meteorologist) said. From then on, his words are only reported and hence they are not in quotes and they have to be translated to Konjunktiv:
Ich sage: "Es ist halb acht."
Ich sage, es sei halb acht.
The first one is myself speaking, the latter is me reporting what I have said before. Using different forms of Konjunktiv allows to express different stances towards what is reported to have been said:
Mir meldet er aus Linz, er läge krank,
Doch hab ich sichre Nachricht, daß er sich
Zu Frauenberg versteckt beim Grafen Gallas.
(Friedrich Schiller, Wallenstein, 2. Aufzug, 1. Auftritt)
Here is a link to a famous in-depth discussion about reported speech by Austrian poet Karl Kraus (in German).
Here, the "läge" instead of the also possible "liege" expresses the doubt the one reporting the speech casts on what he reports.
Notice, that what I have just said is correct but you will probably not hear it being used that way in everyday colloquial talk. It would sound overly formal. Commonly you would rather hear something like that (which is not formally correct at all, mind you):
Ich habe gesagt, daß es halb acht ist.