What is a native language?

In the largest and easternmost Swiss canton of Graubünden, natives speak two varieties of German (68%), at least two of Italian(10%), and no less than six of Romansh (15%). A goodly number also speak English. So, what is the native language of a Graubündner?

The origins of the aboriginal Graubünd race, the Rhaetians, are hidden in the mists of time. Were they Estruscans who moved in from the northern end of the Italian peninusal? Dalmatians from Illyria? Or Semites from the Levant? In any of these scenarios, their original "native" tounge" has long since gone extict, and obviously left no trace behind. Instead, the Rhaetians took up the language of their conquerors--Vulgar Latin, spoken by soliders and merchants of the Roman Empire. The Alpine variety which developed now survives in the form of seventeen distinct dialects in Graubünd and its bordering provinces in Italy. These in turn are grouped respecively under the language headings of Friulian (3), Ladin (9), and Romansh (5).
In addition to the above, there is an "official" standardized form of Romansh, which no one actually speaks as a native language.

So, by the time of the Reformation, for at least 1000 years the Rhaetians had been speaking an utterly foreign language--originally the language of their conquerors, during which time it had fragmented into essentially tribal dialects. Latin remained the language of law, letter, and liturgy: in short, the official language. The Bernese dialect of German served as the language of wider communication.

Enter the Reformation. Martin Luther published the Bible in what would henceforth be known, and taught, as Standard German. The tribal speech of the Teutons had finally gained official status as a language, and as such, eventually replaced Latin altogether. But through a quirk of geopolitics, Romansh, backed by its own seventeenth-century translation of the Bible, emerged as a national language of Switzerland. There was only one problem: no one spoke it! Rhaetians continued to speak at least five different dialects, collectively referred to as Romansh.

To this day, the five official dialects of Romansh continue to be taught in the elementary schools of Graubünden (one of them, in only a single classroom). But the children speak their "native language" with a German accent, because for all but two hours a day (one hour for the official form, one for the dialect? not sure), their instruction is conducted in standard German.

In what sense is a system of speech a native language when it has no monolinguals, no native speakers of its official form, and no chance of surviving another generation without official sponsorship?

It is a native language because a government policy has declared it to be one.
And on that basis alone, it will survive as such as long as does the policy that created it.