1. The fact that there are people of chinese and labanese decent in DR doesn't make DR multi ethnic. Most of them are of 3rd and 4th generation and identify themselves as dominican. Yes they celebrate the culture of their inmigrant ancestors, but they dont consider themselves lebanese, palestine or syrian. Since the chinese are more recent, they still haven't fully blended into the DR population, so they still count as inmigrants. DR is vastly a monocultural population. For the most part, everybody in DR speaks the same language, eats the same type of food and dances to bachata and merengue, no matter where in DR you at, its the same. Dominicans from mountainous areas are lighter than those from the coast, but the cultural differences are very, very minimal.
2.
https://summa.upsa.es/high.raw?id=0000004909&name=00000001.original.pdf
That PDF is a document from a provincial council held in Santo Domingo by spanish authorities in 1622, where the demonym "dominican" is used several times and that is because "dominican" mean't "people native to Santo Domingo" (Sanctus Dominicus in latin) which was the name of the island back in the day. The name was coined by Cristopher Columbus who named the island Santo Domingo in honor of his father "Domingo Colón" or "Dominicus Columbus" and because he arrived in the island on a sunday that was coincidentally the day of "Santo Domingo de Guzman", so the people who were born here were to be called...you guessed it...
dominicans. Which is why i find it very interesting that dominicans love to party on sundays and for some reason its the favorite day of the week.
More information about the word "Dominican":
Source
The problem with "spanish criollos" is that venezuelans, colombians, cubans and puertoricans also refered to themselves as spanish criollos, because back in the day there was very little difference between dominicans, cubans, puertoricans, venezuelans and northern colombians (mostly Cartagena) and were closely related, but the term "dominican" was necessary to specify the inhabitants of the island of Santo Domingo, because spanish criollo could be anybody from Cartagena to Caracas to Havana to Santo Domingo. We call ourselves "criollo" even to this day.