The days of Noach were or was?

Summary: Considering a dispute in the masorah on whether it is vayhi or vayihyu. And whether Onkelos or the Samaritans can shed light on this. (No, they cannot.)

Post: In parashat Noach, we read (9:29):

כט  וַיִּהְיוּ, כָּל-יְמֵי-נֹחַ, תְּשַׁע מֵאוֹת שָׁנָה, וַחֲמִשִּׁים שָׁנָה; וַיָּמֹת.  {פ}

29 And all the days of Noah were nine hundred and fifty years; and he died. {P}

Or do we? In our Mikraos Gedolos, the first word is different:

Targum Onkelos and Targum Pseudo-Jonathan both have vahavo, which would seem to reflect the plural rather than singular form or the verb. Thus, in Onkelos:

ט,כט וַיִּהְיוּ, כָּל-יְמֵי-נֹחַ, תְּשַׁע מֵאוֹת שָׁנָה, וַחֲמִשִּׁים שָׁנָה; וַיָּמֹת.  {פ}

וַהֲווֹ, כָּל יוֹמֵי נוֹחַ, תְּשַׁע מְאָה וְחַמְשִׁין, שְׁנִין; וּמִית.  {פ}

We should have expected vahava rather than vahavo. However, this might well not reflect any real difference. After all, this is translation, and the plural is a better fit for the word yemei. And elsewhere, where all of our masorot are in agreement that it is singular, these Targumim still have vahavo. Thus, by Chanoch at the end of parashat Bereishit:

ה,כג וַיְהִי, כָּל-יְמֵי חֲנוֹךְ, חָמֵשׁ וְשִׁשִּׁים שָׁנָה, וּשְׁלֹשׁ מֵאוֹת שָׁנָה.

וַהֲווֹ, כָּל יוֹמֵי חֲנוֹךְ, תְּלָת מְאָה וְשִׁתִּין וַחֲמֵישׁ, שְׁנִין.

and by Lemech, in the same place:

ה,לא וַיְהִי, כָּל-יְמֵי-לֶמֶךְ, שֶׁבַע וְשִׁבְעִים שָׁנָה, וּשְׁבַע מֵאוֹת שָׁנָה; וַיָּמֹת.  {ס}

וַהֲווֹ, כָּל יוֹמֵי לֶמֶךְ, שְׁבַע מְאָה וְשִׁבְעִין וּשְׁבַע, שְׁנִין; וּמִית.  {ס}

This is also a divergence regarding Noach between the Masoretic text and the Samaritan text, unless of course it isn't.

The text on the left is the Samaritan text. Note that they change vayhi to vayihyu. Once again, I wouldn't put much stock in the Samaritan text. Even in this particular instance, in which it is a machloket within our own masoret, I wouldn't consider this to be good evidence that it was this way in days of old. It is the Samaritan derech to smooth out the text, and in all the other vayhis they also emend the text to vayihyu.

Thus, for Chanoch, they have this switch:

and for Lemech, they have this switch as well.

Not to mention, for all other "days of", it is consistently vayihyu. So while there is an off-chance that the Samaritan text reflects the original, there is no evidence of this, since countless times the Samaritans do just this, emending the text to create a smooth consistent reading.

Here in Minchas Shai's discussion of the matter. He doesn't discuss Onkelos or the Samaritan text, but just competing textual traditions in actual sifrei Torah and masorot:

Thus, two masorot, manifest in different areas, that it is either cha"l or chala"n, that either Chanoch and Lemech are alone in this, or else Noach is among the group as well.

Or Torah also argues in favor of vayhi, discussing it at length.