Denth is Returned, called VaraTreledees by Nightblood, one of the Five Scholars who all were Returned. But he is also a descendant of Vo, the First Returned. He is related to the royal line and Siri and Vivenna. He has Royal Locks independently from his Returned status. WoBs:
Welcome to the Shard!
Kaladin's situation with Syl was a bit different, because he didn't fully break his Oaths, but was very close to it. Shallan did it fully. Shallan can help her, it's very likely that if she reswear her Oaths, Testament will be fully back (which isn't possible for normal Deadeye, as their original Knights are dead, Shallan is alive). And Shallan is already in process of reconstructing her Oaths, I believe "I'm terrified" spoken in WoK was one of Oaths resweared.
And yes, it's possible to bond multiple spren
If you want to read some theories about Shallan's Truths with Testament and her recreating the bond, I will shamelessly promote my own here - https://www.17thshard.com/forum/topic/117629-shallans-true-truths-row/
Killing someone would be a very serious case of "harming a living being", not to mention the fact that in this process a soul is ripped away from a body, which is painful.
alder24's post in Is Denth Returned or related to the Idrian royalty was marked as the answer