Was the Rigvedic Sarasvatī river flowing from the mountains to the sea?

When there is some debate about the date of Rigveda, is often mentioned the first half of stanza RV 7.95.2 in order to prove that the Sarasvatī river was still flowing in its full course from the mountains to the sea:

ékācetat sárasvatī nadī́nāṃ 

śúcir yatī́ giríbhya ā́ samudrā́t 

Jamison and Brereton (2014) translate. "Alone of the rivers, Sarasvatī shows clear, as she goes gleaming from the mountains all the way to the sea." It is strange that they translate acetat as present ("shows clear"), because it is an imperfect, the only case in Rigveda of imperfect of this stem of the verb cit 'to perceive' or 'to become perceptible'. Here they have chosen the second meaning, like Geldner, who translated "Einzig unter den Strömen hat sich Sarasvatī hervorgetan" ("Alone among the streams has Sarasvatī distinguished herself").

It is also possible to translate with a past simple: 

"Alone of rivers, Sarasvatī appeared, going pure from the mountains to the ocean."

However, it is also possible that the meaning is the first one, "to perceive, be attentive, think of", speaking of a personified divine river:

"Alone of rivers, Sarasvatī was attentive (to the prayer), going pure from the mountains to the ocean."

Why do we have this imperfect tense here? In RV 7.96.3 there is instead the present cetati, again with the sacred river as subject.

Now, let's see what follows in the same stanza:

rāyáś cétantī bhúvanasya bhū́rer 

ghr̥tám páyo duduhe nā́huṣāya 

Jamison and Brereton translate: " Taking note of the abundant wealth of the world, she has milked out ghee and milk for the Nāhuṣa."  Geldner: "Reichtum der vielgestaltigen Welt kennend, spendete sie Schmalz und Milch dem Nahusstamm." ("Knowing the richness of the diverse world, she donated lard and milk to the Nahus tribe.") 

Here, we have again the verb cet-, but this time the meaning accepted also by Jamison and Brereton and Geldner is 'taking note, perceiving'. Why should there be this change of meaning in the same stanza? Besides that, Geldner also cites in note Bṛhad Devatā 6.20-24.

There, we find this story (in Macdonell's translation):

"20. King Nāhuṣa in former days wishing to consecrate himself for a thousand years, travelled over this (earth) with a single chariot, saying to all streams:

21. 'I am about to offer sacrifice; bring me shares (for it), either in pairs or singly.' The rivers replied to the king : 'How can we, who have but very little power,

22. bring you all the shares for a sacrificial session lasting a thousand years? Resort to the Sarasvatī: she will bring them for you, Nāhuṣa.'

23. Saying 'So be it,' he quickly went to the river Sarasvatī and she received him and yielded (duduhe) (him) milk (and) ghee.

24. This exceedingly marvellous act of the Sarasvatī towards the king, the son of Varuṇa (Vasiṣṭha) proclaimed with the second (stanza) of the first (of the two hymns, viz. vii. 95. 2)."

From this story, we learn that actually the river Sarasvatī was the only one who was attentive to the request of king Nāhuṣa. Also Sāyaṇa's commentary glosses: "Milked for him: the legend is that king Nāhuṣa, before performing a sacrifice for a thousand years, prayed to Sarasvatī, who gave him butter and water, or milk, sufficient for that period." 

This story was so important and well known that we find it again in the Mahābhārata, this time explaining the full identity of the king, who is the son of Nahuṣa, Yayāti:

MBh IX.40.29cd-30: yayau tīrthaṁ mahābāhur yāyātaṁ pṛthivīpate //

yatra yajñe yayātes tu mahārāja sarasvatī / sarpiḥ payaś ca susrāva nāhuṣasya mahātmanaḥ //

"that great-armed one (Balarāma) went to the Tīrtha of Yayāti, O lord of the earth, where, in the sacrifice of Yayāti, O great king, the Sarasvatī produced clarified butter and milk for the great-souled son of Nahuṣa."

So, if we accept this story as source of this stanza, everything becomes clear, and the river 'flowing from the mountains to the sea', which should be obvious, can actually be a special remark about a time when the Sarasvatī had a perennial course, before 4.5 kya (2550 BCE) according to a recent study. Since Yayāti was at least 30 generations before Divodāsa, a contemporary of Vasiṣṭha and the old Rigveda where RV 7.95 belongs, who lived at the time of the drought around 2000 BCE, we can place Yayāti around 2600 BCE, if we consider him a historical figure. 

PS: I would like to thank VR Patil for his Note on the Rigvedic Verse “Rv-7.95,2” for inspiring this analysis: he is the first to my knowledge who suggested that this verse can refer to the ancient river in a previous condition: "in the present note, we shall restrict our discussion on a verse selected from the hymn Rv-7.95 dedicated to Saraswati wherein the Poet has brought out the pre-Rigvedic condition of the most celebrated river of Vedic People. [...] As far as the term Nahusha/Nahusa is concerned, it is the name of the king who was the descendant of Royal Queen Ila, but lived during the pre-Rigvedic time, and ruled his kingdom from the town located on the bank of river Saraswati. The Poet in the above verse has made it clear that during the early part of Nahusha’s regime, as there was no rain deficiency (please refer to verse Rv-6.49,7 for the designation of river Saraswati as Paviravi Kanya or the child of Lightning which clearly indicates the rain dependency of river), Saraswati used to complete her natural journey from origin to Ocean. Subsequently, when Saraswati came under the influence of demon Vritra (personification of Drought), then the ancestors of Rigvedic Poets introduced a new warrior god Indra to slay Vritra. But the entry of Indra in the Vedic world was strongly opposed by Nahusha as he had full faith in Seven Adityas".