So a few things from that here are my theory's
Oathbringer spoilers removed per OB Spoiler Policy.
What we know a little bit about hemalergy it creates a tear in your soul, stamping a new piece to the host.
What we also know about soul tears is that people have them naturally , crazy people.
Tears also seem to be affected by allomancy at least we know spiked individuals are . Too many and you can be controlled or talked too .
So senario 1 I propose is
Kel convinces Marsh to let him take controller of him too help the people in the South or maybe for other kel reasons .
Marsh maybe agrees, for the idea of him returning being related to the fused he'd have to be willing . that's not likely as we know how Marsh feels about being controlled . But we're dealing with hemalergy and realmantics we don't comprehend, also Kel is Kel , he'll get his way . He hacks the magic anyway to controller Marsh to spiske himself with a kel spike or Kell gets his Lord Mistborn friend to get Marsh to spike himself with a kel spike . Or maybe he just slips right into Marsh because of all those tears in Marsh's soul .
His scars return as they are an aspect of himself his identity. He looks like himself because Marsh and him looked the same.
Maybe Kel spike maybe is harmonium or atium .