The Romans are using the carrot much more often than the stick, since the population of Rhomania in the East is 95% local, 4% mixed, and 1% Imperial heartlander. But Greek is the language of government and commerce, so the advantages of learning Greek and converting to Orthodoxy make for a pretty juicy carrot. So while the rural peasantry are practically untouched by romanization, the upper and middle classes are a different story.
A hundred years ago Korea could field a powerful navy that could go toe to toe with the wokou, but the nomad threat to the north diverted resources. So Korea isn't a total recluse, but virtually all overseas trade is solely with the Cham and it is a fraction of what it was a century ago. Right now it is a muttering vassal of Tieh China, although its mutterings are drowned out by the Cham's significantly louder rumblings.
There are the OTL Hui, and there is Chinese 'court' Islam, whose practioners number in the few thousand at most and isn't considered Muslim even by the Hui.
An important thing to remember is that the Ethiopian attack on Mecca came at the tail end of a jihad. The Ethiopians had spent the last several decades grinding down the Somali, Swahili, Yemeni, and Hedjazi and the attack was Ethiopia's way of saying 'piss off already'. So by the time it happened the local Muslims had already shot their bolt, and when the Mamelukes tried to intervene the Romans threatened to invade.
As for the Roman conquest of Jerusalem, Andreas Niketas killed the Mamelukes in the process. The Marinids did start waging the 'sea jihad' but simple geography meant their victims were mainly Iberians, Arletians, and Italians. The Timurids and Ottomans were too busy glaring at each other, plus the Ottoman Sultan had campaigned briefly against Andreas Niketas and didn't get utterly smashed solely because Andreas didn't go for the kill. And the Ottoman threats against the Omani drove them into the arms of the Ethiopians and Romans for protection.
Those are the specific arguments why there was no mass Muslim reaction to those events. My general argument though is that I don't see why the Muslim world would be more responsive and unified against threats to its holy cities any more than the Christian world was IOTL.
The Hedjaz and the Najd are currently ruled by the House of Saud which took over from the hardliners. For a while under Andreas Niketas they paid a token tribute of a couple of fine Arabian stallions a year to the White Palace but that was quietly dropped during his less scary successors. Their main concern is staying independent, which means not antagonizing their much larger neighbors. The Saudi lean towards the Romans though, since the Ottomans have expressed interest in becoming Sharifs of Mecca.
A lot of the damage done to the Dar al-Islam has been self-inflicted. Four of the five main Muslim powers of 1300 to 1500, the Mamelukes, Ottomans, Jalayirids/Persians, and Timurids expended much of their energy fighting each other. The Romans never had to fight the Ottomans and Mamelukes at the same time, which was good for them considering how hard of a time they had during the Time of Troubles fighting the Abbasids and Ottomans. The fifth are the Marinids, which incidentally can be considered Islam's success story for the time in question.
The standard Triune dialect will be the court dialect, but there will be regional dialects. It'll probably end up like the modern British government, complicated, based on local tradition, and with nothing written down.