• We haven't seen Thaidakar yet, so we don't know if it's an alias or what
• There are ways to overcome the Connection problem and Worldhop, we know this, though we don't know the particulars.
A Metalmind that gives anyone who wears it the ability to tap it (by allowing them to use Investiture and the Investiture not being keyed to any specific individual, which we know is possible) is possible, and can be used by anyone
• We don't know what power Mraize's Aviar has. Possibly Lifesense (he's displayed a sixth sense for detecting people) or mind detection blocker (they seem to be the most common on the Pantheon Isles, the Eelakin and the Homeisles; would be useful in the Cognitive Realm)
• The holes in Shallan's memories... aren't self-inflicted but a result of past trauma, said trauma is connected to Ghostbloods in her past who were connected to the Davar family, so were the Skybreakers. We don't have the full picture yet.
• the Bondsmith equivalent, since the Fused abilities seem similar to one Surge of a Radiant Order, either Tension or Adhesion. Adhesion or binding things seems much closer to Honor, so Adhesion might be the one that's missing
• We don't know yet, not enough information, but we do have a few ideas that you can check out in the particular preview chapter thread as well as one topic in RoW lore & mechanics
• As of now, Nale is the only known Radiant to have sworn the Fifth Ideal, but back before the Day of Recreance there were other Radiants who'd done the same.
I'm assuming you meant to say Fifth Ideal not Fourth but if not, among our current cast, there's no Windrunner of Fourth Oath. Shallan is on her Fourth Truth. Jasnah is hinted to have spoken her Fourth Ideal in Oathbringer.
• Perpendicularities, places where a Shard's power pools naturally and which allows one to travel between the Physical, Cognitive and Spiritual.
• It has something to do with the Rhythms. It seems to utilize Lifespren.
• The Stormfather is also prejudiced against humans because of the Recreance. He's prejudiced against the Singers because they have been on the opposite sides of a very long war
• We don't know where he got that. He said it's not from Roshar or Nalthis. He said Nalthis is too young (which was interesting) and that Roshar might have it but the Highstorm caused crem deposits might also make it difficult to find as the entire landmass is geologically new
• Nightblood could permanently kill a Fused. It seems in the Battle of Thaylen City, he permanently killed a Thunderclast.
Nightblood and Vyre's knife? Hmm... Nightblood destroys across all three Realms and consumes everything, and Vyre's knife traps a Cognitive entity to prevent it from regaining a Physical form
• We haven't had a Jasnah or Hoid PoV in RoW yet, so no confirmation only rumours as per Navani
• Urithiru reacts to the Highstorm by manipulating its temperature and pressure, even while the Sibling is "sleeping", beyond that we don't know
The Navani and Melishi predictions are quite popular, and yeah the metal could be Chromium (Allomantic ability of Leeching) or Nicrosil (Feruchemical ability of storing Investiture)