To the best of my knowledge, the only two known ways to get every ability in the game are through the use of Manipulate Form or Illithid Savant.
Here's a third way to do it that makes use of neither.
First, how to do it as a psion.
Step 1: Be at least level 17 and know the power Greater Metamorphosis.
Step 2: Use Greater Metamorphosis to change into a Lilitu (FC1, page 43).
Step 3: Use your Item Use (Ex) ability to use a scroll of gate to gate in a Solar.
Step 4: Order the Solar to Wish you up a scroll of Ice Assassin of a Psionic Elder Brain with the powers Fusion, Psionic Restoration, Mindwipe, and Astral Seed on his powers known list (as it's a scroll this gets around the need for a material component for Ice Assassin)
Step 5: Use your Item Use ability to activate the scroll.
Step 6: Catch 3 humanoid creatures of at least 8 HD and paralyze them, bring them down to 1 HP, and place them by you.
Step 7: Order your IA Elder Brain to use Fusion with you and to immediately relinquish control to you one it is manifested.
Step 8: Use Greater Metamorphosis to turn into a Barghest.
Step 9: Use Mindwipe on yourself to give yourself 19 negative levels (might require repeated manifestations).
Step 10: Use your Feed ability to consume the humanoids from step 6.
Step 11: Manifest Psionic Restoration to clear away your negative levels.
Step 12: Manifest Astral Seed.
Step 13: Kill yourself.
Step 14: Rebuild your body, you now have all the abilities of the fusion (including Ex, SU, and SP).
Step 15: Use your Item Use ability to gate in another Solar and this time have it Wish up a scroll of Ice Assassin of an Elder Phaerimm with the spells Ice Assassin, and Wish on it's list of spells known.
Step 16: Use the IA scroll.
Step 17: Repeat the above to gain all of the Phaerimm's abilities, most especially it's ability to cast as a 19th level Sorcerer and have it's spells be treated as spell like abilities.
Step 18: Create IA's of every creature and class, repeating the process as necessary and using shapechange to get the monsters that wouldn't qualify for Fusion to qualify.
Once you have done all that, create an IA of a Greater Deity. Use Fusion on it and then kill yourself after manifesting Astral Seed. You loose an HD (can't hit deities with negative levels) but that is a small price to pay for 20 divine ranks.
The last IA should be an Aleax Ice Assassin of your original body (unless you know longer want that base form, in which case choose whichever base form you want instead). Have it use it's shapechange ability to loose the construct type by shifting into the base form you want. Use Fusion with it (taking on it's physical form) and then cast an epic spell that is loaded down with backlash damage, as much as you can. This is the only way to kill an Aleax who's singular enemy isn't around. Once you remake your body you will have the base form you want, multiple copies of every single ability in the game, 20 divine ranks, and be totally immune to all harm except Epic spell backlash damage.
If you want to pick up Manipulate Form and Illithid Savant, feel free for the complete set but you don't need to use them in any way.
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I call this the Path of Ascension.