So, I was wondering how Lightweaving could exist on both Sel and Roshar. I mean, Brandon's never put a magic system on two shardworlds before!
Answer: Hemalurgy.
Hemalurgy takes Investitures from something and bestows it on something else. In the process, the Investiture becomes a bit weaker. This developes my theory that lightweaving is originally from Sel, but eventually came to Roshar via a Worldhopper Hemalurgist. For example, if Hoid or Nazh went to Sel and took a Lightweaver's power, then gave it to a person on Roshar, they would gain Lightweaving. However, it would be weaker, which explains why Roshar's version of Lightweaving is harder to do.
If Lightweaving can be hereditary, or if Shallan has a bit of metal in her somewhere, this all makes sense.