Is an undead considered a “Living creature”?

In D&D, making multiple attacks with weapons, fists, claws, horns, and the like is usually done via the Extra Attack feature and the Multiattack action

PC has Helm of Comprehend Languages. He wargs into his familiar Owl. Owl flies ahead, sees writing in Infernal. Can the PC read that via the Helm?

The Horn of Valhalla summons Beserkers However, as the text describes them as spirits, could/should they be treated as Undead?

The Echo Knight’s echo, does it count as a creature for the purpose of optional flanking rules and as a target to spells and attacks?

I see that features from Way of the Sun Soul has different writing on Xanathar’s

I wasn’t aware that negative and positive energy are not considered magical. Would you apply the same rules to Negative Energy Flood?

Is the Pride Silk Outfit actually armor? As in you need Light Armor Proficiency to wear it?