So what have we learned about Kızderbent?
The people who lived in Kızderbent speak a Slavonic-based language, called Trakatroukika, with a significant Turkish admixture. The Turkish admixture is definitely there in the vocabulary; I don't have enough information yet on whether the admixture also extended to the grammar, making Trakatroukika a true mixed language. The basic vocabulary is still Slavonic; that only the numbers 1–4 remain Slavonic is consistent with the pervasive influence of Turkish as the trade language. (Rural populations only need large numbers for commerce.)
There are claims of Armenian elements in Trakatroukika, but they are made in passing, and no evidence has turned up of such influence. My guess is that Armenians lived in the village, and authors lazily assumed Armenian had made it into the language mix. There is some Greek presence in Trakatroukika, but it is underwhelming; outside of cultural loans, there are two cooking terms, "thank you", and two place names. I think these show contact with Greek-speakers, maybe even contact where the Trakatroukides came from; but they do not show extensive presence of Greek-speakers in Kızderbent. There is a slight presence of Albanian in the vocabulary.
Kızderbent was an affluent village on a major trade route; the village was initially established to guard the route, and enjoyed tax exemptions. It was settled sometime between the 16th and 18th century—likelier earlier than later. This makes it roughly contemporary to the other waves of settlement in Bithynia, by Greeks (including Tsakonians) and Bulgarians; the Balkan peoples of the Ottoman Empire were mobile, and there were many landed settlements in the parts of present-day Turkey closest to the Balkans (Eastern Thrace, Bithynia; in fact the Greek of the Western Asia Minor coast has been argued to be a resettlement in Ottoman times from the Greek islands, and its dialect is quite close to the dialect of Crete and the Cyclades.)
The village appears to have been isolated from other Bulgarian-speaking villages, though we know nothing of the nearby Pamuk-Dervent. Early 19th century accounts of the village call it either entirely Bulgarian (Salvatore, Tancoigne, Keppel) or entirely Greek (Irby & Mangles, Leake, Conder citing Leake). Given that Ottoman identity was credal, and there was no ecclesiastical autonomy for Bulgarians at the time, this is still consistent with Greek not being spoken in Kızderbent.
The linguistic shift of Kızderbent towards Turkish is mentioned several times, and there also appears to have been a cultural shift (Anatolian dances, songs in Turkish, oil wrestling.) It has been suggested that the shift to Turkish language and cultural practices but Greek identity means Kızderbent was influenced by Turkish-speaking Christians, who identified as Greek.
With the imposition of 19th century nationalisms, the Slavophone villages of Asia Minor had to choose whether to identify as Bulgarian or Greek: there was clearly disagreement in many of the villages, with Hellenising Slavophones derided as Grecomans—as occurred in the Balkans. We have no evidence that the dispute translated into violence as it did in the Balkans; but with the wars of the early 20th century, these populations were made to move on, not to where the settlers came from, but to where the settlers now identified with—and where the countries receiving them could find room for them.
So the majority of Trakatroukides ended up scattered through Greek Macedonia and Western Thrace after 1922 (after Kızderbent was burned down by Turkish irregulars in 1920), and a minority of Trakatroukides had already settled in Bulgaria by 1891. (So there was disagreement about identity in Kızderbent as well.) Kızderbent is now inhabited by Turkish refugees from the Kavala region, and the broader region had already been settled by Muslim refugees by 1919. There are several more such stories: Patriarchists coming to Greece from Bulgaria (Petrich > Neo Petritsi); Southern Albania > Eastern Thrace > Serres prefecture; Kostur/Kastoria > Stengelköy > Degeagaç/Alexandroupoli > Bulgaria; and the intriguing story pointed out by Anon of the Gallipoli Serbs.
(Robert Greenberg on Ivić in his 1957 study trying to prove the Slavs of Gallipoli were Serbs; Henning Anderson mentioning Ivić's data in a purely linguistic context, although the pitch accent described is indeed characteristic of Serbo-Croat and not Bulgaro-Macedonian; Website claiming them for Greece—and commenter claiming them for Serbia—with some indication that a Greek identity is being cultivated in their new home of Pehčevo, then Serbia, now FYROM; Greek blog post on same.)
The local tradition is that the Slavonic base of Trakatroukika is from Ohrid: this would make Kızderbent distinct from the other Slavonic-speaking settlements of Bithynia, which are from Haskovo, and it would mean that like Stengelköy, the Slavonic base is Macedonian and not Bulgarian. I don't know what the linguistic evidence says (the word lists on the Motley Word, the YouTube recording), but any speaker of Bulgarian would be able to tell immediately. I have not seen a serious counterproposal for a Bulgarian place of origin of Trakatroukika (Jireček's guess of Momim Prohod is not serious); the few Albanian words of Trakatroukika, though, also point West rather than East for its origins. Trakatroukika is mutually intelligible with the Slavonic of Kilkis and Thessalonica prefectures (though also clearly not identical to it: voda ~ odam); but mutual intelligibility across the Bulgaro-Macedonian continuum is possible, so it does not prove one way or another.
Trakatroukika survived a generation, though the youngest speakers would be the children of the refugee generation, and past middle age by now. Young Trakatroukides have some emblematic phrases of Trakatroukika. A sense of Trakatroukis identity persists, and Kızderbent folklore (dance troups, cultural associations) has been cultivated over the past three decades. Indigenous Macedonian Slavonic speakers and Trakatroukides coexist and have intermarried in villages like Polypetro and Nikomidino (though not Valtotopi—which prides itself on being purely from Kızderbent.) Polypetro seems to make more of its Kızderbent roots than its indigenous roots, and there are complexities of identity and identification there that I can only guess at.