Basic Guide for how to solo MotB, devouring all your companions and crafting the Amulet of Betrayal Personified

Try to maintain a Neutral alignment. Evil characters won't be able to get Eternal Rest. Good characters won't be able to do some of the nasty things you'll have to do.
You can keep your alignment in check by using Spirit Gorge (annoying as it increases craving) for Evil points, and Eternal Rest (much more convenient) for Good points.
Epic Resilience is very important, as you will be making a lot of saving throws against permanent debuffs and you can't rely on your allies to remove them.
An item with Immunity to Level Drain is very important.
Be sure to Devour Gulk'aush, as her essence can be used to make the Amulet of Pure Love, which may be your only way to cast Restoration and remove debuffs.
Fights will last a long time since you just don't put out the damage of a full-sized party, so play a build with high AC and saving throws. I used a Sorcerer/Dragon Disciple/Blackguard/Eldritch Knight (a variant on http://nwn2.wikia.com/wiki/Sorcerous_Dragon_Knight_(Sor_11/Pal_5/EK_10/RDD_4)) with early Divine Shield, and Epic Resilience.
Knockdown will be your bane. Many monsters have it and it's hard to defend against.
I did allow my companions to Craft items for me.

You can obtain all 5 companion essences:
1) Do not enter the Death God's Vault until Act II.
2) Pick up Gann and relentlessly apply negative influence. You will need to hit every possible negative influence point with Gann in order for him to rebel at the right time. See below for the available list of -ve influence. You must make sure never to gain +ve Influence with Gann.
3) Devour Okku at the end of Act I
4) Travel to the Wells of Lurue to learn Devour Soul from the Uthraki
5) Enter the Death God's Vault and goad Kaelyn into a fight the opening conversation. The only other alternative to devour Kaelyn comes at the very end, and only if you decide to fight against the Second Crusade. Easier to get it out of the way here first.
5) Get Eternal Rest (only available if you are not Evil) and One of Many.
6) Pursue the usual Act 2 quests. Devour the Wood Man to get Spirit Gorge. Hit all of Gann's -ve Influence effects.
7) There are enough opportunities for Safiya's influence that you can easily raise her to 75 (Devoted) and then immediately begin tearing her down. Praising Artesh for mocking her is a good way to get Safiya very angry with you.
8) Devour Gann in the Skein when he tries to leave.
9) During the Act II Finale, allow One of Many to devour Myrkul. By this point you should be able to get his Devotion feat.
10) Attempt to devour the Founder. Unless Safiya loves you greatly at this point, she will turn on you and you can devour her.
11) Once you are alone with One of Many, he will transform into Myrkul and attempt to kill you. Defeat him and then devour him during the dialog.
12) Forge the Amulet of Betrayal Personified before passing through the Black Gate.

Mapping Gann -ve influence
-6 during recruitment conversation
-6 when you talk to him in general
-6 with Darovik in the Temple of Kelemvor
-6 with Janiik first time (Lurue)
-6 with Janiik second time (Lurue)
-6 with the Bears (Lurue)
-6 in the dreamscape (Lurue)
-6 with the Bheur (Immil Vale)
-6 cutting into the Fentomy conversation (Lakeshore/Nightshore)
-15 offer Vlooshk a taste of Gann (Coveya Kurgannis)
-11 talk Gann down to the Mistress (Coveya Kurgannis)
At this point he should turn rebellious
-1 suggest disposing of the Sleeper (Skein)
At this point he should insist on leaving, and you can kill him.

More -ve Influence with Gann is possible if you keep Kaelyn alive, but this makes it much harder to devour Kaelyn.

I've done a similar playthrough before while maintaining an evil alignment the whole way through. My experience is that Eternal Rest is completely unnecessary. Suppress has no alignment ramifications and is completely self-sufficient unless you decide to devour Myrkul (which will just require a bit of XP offerings until your cravings get back under control). I did my playthrough as a Paladin/Blackguard/Cleric; I'd agree that high AC and saves is definitely the way to go.