I was hoping someone else mighty have some answers for you by now, but i'll add my own observations for what they are worth. This does indeed seem to be a very mixed bag of cultural styles and i would only be guessing at how or why they came together. The sheath itself seems to be a bit of an enigma in and of itself. It is not expertly carved and does not seem to quite fit into styles i am familiar with. But sheaths do sometimes get carved by the people less skilled and knowledgeable of styles and often do not quite fit or hit the mold or model their maker was aiming for. The designs on the bands do not look like anything i have seen that i could say are traditional to Sumatra (or Madura or Bali).
The janggelan hilt could be East Jawa or Madura. They are technically the same, but not from elsewhere in Jawa. Same for the mendak i believe.
The blade does indeed seem to be in the style of Bali, but it raises questions for me. The wear on the blade is oddly uneven. It has very sharp features at the lambe gajah, greneng and front edge of the pejetan, but there is much greater deterioration along the blade's edge, especially towards the tip. What would cause such uneven erosion on a blade such as this? There is also that diagonal feature near the tip of the gonjo which i don't believe i've seen on Bali keris before. So i wonder if this is not of more recent, perhaps Madurese, manufacture and artificially aged.
