Cursed items are not leechable. You'll miss out on cursed indestructible amulets/rings of something if you destroy items by that method, which can be a good source of (magic/elemental) essences.
those however are rare and so it doesn't matter much
What is this "flipping". A few people talk about it in various posts, but I don't know what it actually means.
when you read a scroll of remove curse (whichever is strong enough to get rid of the curse) it has a chance to flip the negatives on the ego item (make them positive) it doesnt affect the positive modifiers though, just the negative ones. This chance is based on your character level (I believe at lvl 50 you have 1/3rd of a chance, less for lower levels). You cannot flip randarts or truearts.
Edit: Another question. It also seems that I cannot extract or recharge the "Digging Wand of Thrain". Is this because its an artifact and thus exempt from all mid level (25) alchemist stuff.
yes. all artifacts cannot be extracted from so it is immune to alchemy... however you can still pump it full of charges via recharge (non-alchemy) because it is an artifact and cant be destroyed.
Oliphant am I, and I never lie.