Radiancy is more than just finding people with the right qualities, it's also about finding people who might need saving, like Teft or Venli, remember that the spren are also sapient beings who are not just mechanically looking for the perfect Radiant (though some of them do have certain systems in place for Bonding). So much also depends on perception, there could be crazy Radiants who interpret the Oaths in a very weird way and the powers would still work. The Ideals of the Lightweavers is really well represented by the names of their Surges: Illumination & Transformation. They illuminate people to their better selves, provide a sort of spiritual sustenance to help transform them to be better versions of themselves. Shallan did help her family in this respect, as noted by her brothers, with her joking and general happiness, same with Tien. And apparently her family was affected by an Unmade.
Nope, corrupted Investiture is red but not everything that is red is corrupted Investiture.
All magics on Sel are region-locked. Their Shards are locked in the Cognitive Realm so none of the magics work off-world... yet. (There are always hacks)
Shai would need to know a lot of the Recreance and of the spren, as well as a lot of Investiture to do anything close to that. Even then, it might not take.
What Chromium does is still not really known, do they burn off the Connection? the Investiture? the metals themselves? Besides, it might end up harming the spren more if it does either of the two former options and nothing for the latter