They came from Sibi, an Afghan territory in 15th century. Langehs of nowadays would naturally identify themeselves with jats with whom they have totally mixed. Your example of niazis doesnt make any sense 1) first of all they are contineous with pashtun belt without any interruption or barriar, they are part of pashtun belt 2) secondly isa khel is geographically part of pakhtunkhwa 3) thirdly they live as a whole tribe, being majority in the area. Before 16th century, not just isa khel, but adjacent lakki and tank were also niazi territories as described in baburnama.
You are going against your own sources. And just checked Sibi is in northen eastern part Balochistan near Seraiki areas. I though they came from Afghanistan? I think this again proves they are indeed Seraiki tribe. Dozens of sources can't be wrong at the same time. If UP pashtuns can maintain their identity then i don't see why not Langahs. And no pashtuns will not identify with jats, does not make any sense.