The Talmud (Berachot 8b-9a) tell us that the Halacha is according to the opinion that, at least if forced to, one can fulfill his obligation for the night time reading of Shema after dawn before sunrise, as some people are still sleeping then. The Talmud adds, though, that in this case one wouldn't recite the blessing of Hashkiveinu. (See Rambam Shema 1:10, Tur/ShA OC 235:4, etc.)
Rashi (Berachot 9b sv. Uvilvad) explains that even just before dawn one should not recite Hashkiveinu because it is already the end of the "lying down" period, not the beginning, so we do not need to pray for lying down ("השכיבנו" from .ש.כ.ב "to lie down"). Tosfot (ibid.) disagrees, arguing that only after dawn is Hashkiveinu omitted because then, as since dawn has already come, it's not fully a time of lying down. The Rambam too only proscribes Hashkiveinu after dawn, and the Tur and Shulchan Arukh agree quoting Tosfot's reasoning.
Elsewhere, Rabbeinu Yonah (Berachot R1a-b) describes a proposal of his for how to conduct oneself at a Maariv Minyan occurring after sunset but before the time for Keriat Shema at night has arrived. (This is a Halachically relevant proposal as it appears to be the one cited in Shulchan Arukh (OC 235:1; see it quoted among other options in the Beit Yosef there).) He posits that the blessings on the Shema can be said with the Minyan and the Shema recited later after nightfall, explaining as follows:
the blessing "HaMaariv Aravim" can be said after sunset before nightfall because the phrase in it גולל אור מפני חשך applies already after sunset;
the blessing "Ga'al Yisrael" which accomplishes the goal of recalling the Exodus at night, need not be said at night proper, for 1) the obligation to recall it at night is not learned from the word "night" but from an extra word in the verse (כל ימי חייך -- הלילות), 2) we see from the case of reciting the nighttime Keriat Shema after dawn that "Ga'al Yisrael" is said, so it can certainly be said after sunset before nightfall;
the blessing of "Yir'u Eineinu" and its associated verses are but a custom so we need not be particular about their timing.
He does not once mention the blessing of Hashkiveinu. Moreover, he interjects after discussing the blessing "Ga'al Yisrael" to note that one shouldn't think that just as between dawn and sunrise is considered, at least if forced to, both a time of rising and a time of lying down (and hence either the morning or the nighttime Keriat Shema (and either the morning of nighttime blessings of Ga'al Yisrael) can, at least if forced to, be recited then), that so too between sunset and nightfall has the same status. Rather, he says, that between sunset and nightfall isn't a time of lying down for anyone and thus even if forced to one cannot recite the nighttime Keriat Shema then.
It seems obvious to me that according to this (and at least according to Tosfot as opposed to Rashi, above) if Hashkiveinu is omitted between dawn and sunrise because it is not sufficiently a time of lying down, all the more so it should be omitted between sunset and nightfall, which would not be considered a time of lying down at all.
So my question is: does Rabbeinu Yonah endorse saying Hashkiveinu between sunset and nightfall, and if so, on what basis? If that basis works only according to an opinion we don't rule like (eg. Rashi above about "the end of the 'lying down' period"), has anyone discussed the applicability of Rabbeinu Yonah's ruling (presented in the Shulchan Arukh OC 235:1) to us in light of that?