Is there any way that Transylvania could've, after 1989, been torn apart by ethnic cleansing between Hungarians & Romanians along the scale of the FY Wars ?
Difficult to believe. Romanians are an overall majority in the region and the two counties where Hungarians are majoritarian are surrounded by Romanian-majoritarian counties.
Somehow make the 1956 Hungarian Revolution successful. Then, cum the collapse of Soviet power in Eastern Europe, a hypernationalist Hungary (perhaps a series of scandals discredit all the major parties?) eagerly invades the lost territory in order to reunite the motherland.