The so-called Doloneia has been debated for some time, since antiquity in fact. There isn't really a consensus, though most scholars consider it an interpolation. The tradition that the Doloneia is an interpolation goes back to a Homeric scholiast in antiquity, who attributed the Doloneia to Pisistratus. More likely, if the scholiast is right, the Doloneia was part of an alternate version of the Iliad (there were many such versions surviving well into antiquity, even after the relative standardization of the text by the Alexandrian scholars) that was added by Pisistratus, possibly as part of the so-called Pisistratean Recension (if it existed). Certainly the passage is suspect, as it lacks certain distinctive features of the rest of the poem besides not really making sense, but it's been pointed out that the Homeric Poems were the result of a centuries-old oral poetic tradition, and that anomalies shouldn't be unexpected. Many such anomalies were culled from the text by the Alexandrian scholars as interpolations, some of them quite long, and there are lots of passages in Homer that certainly look like interpolations but may well be genuine products of the oral tradition (I dunno, I'm not really a Homeric scholar, they get all tied up in knots about this sort of thing). But the Doloneia is unusually long and if it is an interpolation it probably represents a fairly early one, probably inserted from another part of the epic cycle--the Doloneia appears to be linked to an epic tradition surrounding Rhesus, who is of course murdered in his sleep in the passage, that we know existed

So my theory is it was written in the 5th to 4th century B.C. The reason behind this is the character Dolan. In my opinion a character like him would have been very popular to add due to the traitor at Thermopylae, Ephialtes of Trachis, in 480 B.C.

I'm not so sure about this. I've never heard any connection supposed between Ephialtes and Dolon, they seem to me to have very little to do with each other. I mean, Dolon exists literally to be killed--the New Pauly suggests that maybe he was an invention of the author of the Doloneia, who created a character to link a tradition about a night expedition by Diomedes and Odysseus with the death of Rhesus. His name is certainly suspect (Δόλων isn't really a name, its a derived form from δόλος, "craft, cunning") and his lack of any personality or importance despite taking up so much of the book is more than a bit odd if the character existed in the epic cycle before his inclusion in Homer. Ephialtes is certainly too late to be an influence on the Doloneia, as it appears to have been named very early, attributed to Pisistratus by the scholiasts and possibly known to Herodotus. It lacks features common to the rest of the poem, but its language still appears to be close enough to Homer's that it's probably at least in some way derived from some part of the epic cycle. The New Pauly's explanation of the passage as an interpolation of Rhesus' part of the epic cycle into Homer is as convincing I think as any, but there are lots of alternate suggestions out there and there's still not even a consensus on whether it's an interpolation or not