Because damage remains marked on creatures until the damage is removed as the turn ends, nonlethal damage dealt to creatures you control may become lethal if Leyline of Hope leaves the battlefield or if your life total drops below 7 life more than your starting life total
Unlike those cards however, its only been printed at base rarity in two sets (Scarlet & Violet Base Set and Obsidian Flames), so even the non-Reverse Holo versions are remarkably valuable
That creature is just gone (unless its a commander)
Historic itself is used as an adjective to describe other things, so on cards, you'll see phrases like "a historic spell," "a historic permanent," or on cards like Abstergo Entertainment, "a historic card." Just remember that if something refers to casting a historic spell, Abstergo Entertainment won't help you, as playing a land is not casting a spell
Read the Bones sees a bit of Commander play, but its not a staple like the previous Furby Bonus Cards
You hope to never need the second replacement effect because it'll rarely take effect and gain you life