Hi!
Here's my demand:

I've got a group of 2 players wich one's character managed to die. I planned for each of them their own little solo adventure: one is to find a way to resurect her friend, the other to ventures in the land of death.
Problem is the living one's player is always either unreachable, too tired or has no time to play. We decided to not wait after him to play the rest of the campaign.

So my only player's character is dead. I apply some rule of the Ghostwalk book on her.

I try to find ideas for her to get back to life, or in the worse case, undeath, or a least a way to buy some time before her soul to pass through the Veil.

The setting is a desert - egyptian mythos/asian style.*
*She is a female loyal neutral elf dread necromancer lv 5 who work undercover as a librarian in a temple full of good and lawful monksand priests. She died attacked by a band of nomad scavenger in the waste. She is now pursued by Anubis's hounds who track the lost souls in etheral plane that didn't follow the Calling and prevent them for being turned by undead or falling in the Shadow Plane by sending them to the obelisks, that send them like the Calling to the Judgement of the Soul etc etc..

I ask your help, any ideas are welcome!

I think keeping the three options open for her to pass through the veil, become undead or back to life would give the character the chance to pick her destiny. If I were playing this character, I would like the opportunity to experience the option of deciding what road I would travel. Now most people would want to become alive again and rightly so would I so I guess making the challenges more demanding or maybe consecutive. There is a chance that passing into undeath she may grow in power and become an intelligent undead such as a lich who's story now changes to trying to find immortality in the mortal world, that is once she makes it through to the mortal realm once again. Passing through the veil I understand is heaven so to speak, so maybe gifting her with angelic earned class levels could help in this transformation. Maybe taking the time to meet a npc such as a gatekeeper who challenges the players in obtaining a relic of importance for the npc only for a cliff hanger storyline to happen. It seems that you have much fleshed out so I guess more could be revealed.

You could figure out some storylines to give the choices real weight. Maybe she meets other souls who are in similar dire straits, and will have to choose to leave others to their fates to be reborn, or sacrifice herself to give them the chance to go on, etc.

Maybe the land of the dead is in such turmoil that important and rewarding things need to be done there before returning to life.