I usually RP that while Light does mend the flesh and heal undead, it does feel incredibly painful. Like bathing in rubbing alcohol without skin sorts of painful.
Lore-wise Undead can be mended with the light cause it mends their flesh or it can be used to burn their flesh back together.
If an Undead is mended with the light due to their broken bound between their soul and body it will return the lost pain their body has lost so mending will be painful and uncomfortable, although some can endure it and even call upon it in their undeath due to their beliefs.
In terms of Shadow it is said it causes pain as it heals or the way I prefer to see it, it accelerates healing process and with it the pain and forces some extra pain cause void.
In terms of Undead they can’t feel pain… so most of them prefer shadow or void, I don’t believe there is a real explanation to that.
Using the Light really is just a matter of willpower. Believe hard enough, and it’ll answer your call - no matter if undead or living.
It does, and if we go by game mechanics, it can heal the living too. Priests have an ability called Shadow Mend that heals you, and then damages you if you aren’t out of combat by the time it finishes.
In regards to your question: I don’t think the Shadow can restore entire body parts. It’s never been shown and it’s also admittedly a little boring if it could, purely because it takes away any impact the loss of an arm may have. For Forsaken, they could just sew on a new one.
Using the light in lore has two methods, be part of it (Paladin), or use it (Priest). The light in use can be something they can wield easier if it ‘operates’ on your belief of cause and reason, and the will behind it (from my understanding). Just see Scarlet Crusade, deeds of evil, yet the light adheres, or seek Army of Light, who before forced people to become part of them, and the light still adhered. And then Yrel’s people, creating a holy crusade of her own in the name of purity, killing or converting anyone in their way, and yet the light adheres.
The Undead are known to be able to be healed by the light, with the cost of pain.
They can be healed with certain shadow spells.
If their limp is too damaged, they are known just to get a replacement. According to books, there is a business selling spare parts to the Undead who seeks a new or better part, as well as services offered for fixing stitches or restitch parts.
“Thank you for choosing Arcane Magic as your healing method, side effects might include: limbs glowing a strange colour, animal limbs growing instead of humanoid ones, limbs becoming either crystallised or elemental versions of themselves, limbs accidently causing pocket-portals which might randomly teleport you without warning. Thank you and have a nice day!”