I spent a good while sifting through the histories in my Uni library wondering if this was something I'd passed over in all the books I've read on Tibetan history, and maybe if some books could go into detail about it.

I checked every book that might possibly mention it, even Tibet and the People's Republic of China which uses as it's first chapter "Chinese Justification from Chinese Sources," which did not mention the 16-Article Code as far as I could tell. I checked through anthologies of Tibetan sources that discuss primary sources of law-and-order in Tibet, including a murder trial from the early medieval period, and couldn't find a single mention of the Sixteen (or Fifteen) Article Code.

That said, it's logically impossible to prove a negative, and it would only take a single document to prove me wrong if I were to insist that this Code doesn't exist. However I can walk you through this Wikipedia article and the notion of "justification" you mentioned.

The 16-Article Code was created in the early 17th century and effective in Tibet until 1959.

Assuming the Code exists, then it's certainly possible it was effective in Tibet until 1959. Given how grevious the things written in the Code are, I struggle with why it was in place until '59 after the Chinese had been in power for almost a decade. It makes sense that portions of it might remain in effect, but it's worth considering that it's the sort of thing that the Communist People's Liberation Army would have done away with around the time of the Seventeen Point Agreement.

It was created during the Garma Karmapa regime, it covers military, civil and administrative affairs, also degrees of punishment.

This phrase "Garma Karmapa regime" is the most telling and confusing part of trying to identify exactly what the 16 Article Code is supposed to refer to. The Karmapa Lamas never held political power in Tibet in the way that the Dalai Lamas did. So the Karmapas, while they certainly held localized power over their many manor-like establishments, they were not rulers of a political "regime."

The Karmapas did obviously hold significant soft power as the primary religious teachers of three Tibetan and one Chinese dynasty. While China was ruled by the Ming, Tibet went through a succession of three dynasties, the Pagmodrupa, the Rinpungpa, and the Tsangpa. The Karmapas were certainly powerful with all of that access to Kings and Emperors, but they weren't rulers.

The Tsangpa Kings of Tibet spent the entirety of their time at war with the Rinpungpa and then the Qoshot Mongols. The Karmapas were mostly out of the game in the first half of the 17th Century. The 9th Karmapa died in 1603, and his successor wasn't foud until a year later, and wouldn't attain his majority until 1622. By then the Tsangpa's were well entrenched in their seat of power and were tied together with the Karmapa lineage.

So the regime would have been the Tsangpa King's in strictly political terms. It's possible that the article is referring to "the time" of the Karmapa Lama in place of "regime." But it still heavily implies that the Karmapa Lama ruled Tibet (which he didn't) and that the Code applied to all of Tibet as a result. The Karmapa Lamas never held that much political power, and while it's not impossible, I find it hard to believe the Tsangpa Kings had time to rewrite an entirely new and harsh law code in the middle of a progressively worse military situation on the advice of a child under their protection.

Of course, this is Wikipedia, and it's entirely possible that all this information is wrong but that the Code still exists. If that's the case, then it's worth actually looking at Tibet less as a single unified country (this Tibet basically only exists in the minds of non-Tibetans who think in terms of nationalistically whole countries) but as a loose collection of linguistically and religiously similar places. Say a Karmapa Lama wrote the 16 Article Code (sure, the 9th Karmapa could have written it in the last three years of his life and still be a 17th Century law code) it would have only applied (and been effective) in lands the Karmapa Lama ruled. The Karmapa Lama never ruled anything more than his monastic estates, so it would have only applied there. And after 1642, when Kagyu Buddhism was suppressed through U-Tsang (the historical region of Central Tibet) the Karmapa's rule wouldn't have applied in the lands we generally refer to as Tibet at all. (Amdo, Kham, Bhutan, Ladakh, and Nepal, where Karmapa monasteries still existed after the 17th Century, were even less likely to tolerate a centrally imposed law code, frankly.)

After 1642, the Dalai Lamas were swept into power by the Qoshot Mongols and their Geluk sect began suppressing non-Geluk authority throughout Central Tibet. The Jonang School was completely annihilated, the Nyingmapa were mildly tolerated, the Sakya pushed back into their monasteries, and the Kagyu driven to the extremities of the Tibetan Cultural area. Flights of Kagyu Lamas spread southwards to Ladakh, Mustang, and Bhutan. The Karmapa ended up visiting all these regions but was mostly in the northeastern section of Tibet.

The example of Bhutan is illustrative: the Zhabdrung who claimed Ralung Monastery south of Lhasa was driven out of Tibet in the rising tide of Mongol and Geluk power where he established an exile regime. He fled in 1616, and by 1623 he began to consolidate his hold over the country by constructing fortress-monasteries called "Dzongs" in the western half of the country. After 1642, he hoped to reconcile with the Dalai Lama and return to Ralung, but the Dalai Lama sided with the Zhabdrung's opponent (happened to be his cousin) and Ralung stayed firmly in Tibetan control. Bhutan developed as a "Ralung-in-Exile" with all of its legal and administrative structures carried over from before. Where Ralung had switched firmly over to a reincarnate lineage (for example) the Zhabdrung was intent on keeping his exile regime a primogeniture one.

Speculation regarding this document as propaganda: The Karmapas were close with the Ming Emperors, as mentioned. And while many English-language histories promote the idea of the Dalai Lama as the former monarch of Tibet, historically there's some dispute over the legality of this. Their main rivals were the Panchen Lamas, who were also historically close to the Chinese Emperors (of the Qing variety). And in the 20th Century, an open dispute between the Dalai and Panchen Lamas resulted in the Panchen Lama fleeing to China for refuge from the Dalai Lama's growing military power. Shortly afterward, the Chinese Republic adopted a policy of marching the Panchen Lama to his estates in Shigatse (somewhat ironically built by the First Dalai Lama in the 15th Century) at the head of an army. To this day, Chinese Language histories of Tibet promote the Panchen Lama as equal or superior to the Dalai Lamas.

This concept of a "Garma Karmapa regime" might be in earlier pages of these aforementioned Chinese-language histories of Tibet, promoting another Lama as superior, or at the very least, equal to the Dalai Lama in historical power. Since the Karmapas were close contemporaries of the Ming Emperors, it's perfectly reasonable (knowing how myopic Chinese dynastic histories and their government sponsored successors tend to be...) to conclude that the Chinese authors might be trying to promote the Karmapas as predecessors to Geluk power in Tibet, while also pointing out the bougeous control of the Lamas. Two birds with one stone: discredit the Geluk, the main political rival, and the Buddhist Lama hierarchy as a whole, the main cultural-ideological rival in Tibet.

I think the veracity of the document itself is pretty low in priority here. If it did exist, it only applied to an increasingly tiny region for a very short amount of time (certainly not through 1959 in any way that would constitute "Tibet" in all its vastness).

Cont'd.