To start with, Ascension heavily favors PC warriors (notice that the vast majority of the gained abilities give a Miscast Magic percentage but absolutely no penalties to THAC0 or AC or anything a warrior would find useful).
In answer to your question, people have done it soloing a mage or a sorcerer so yes, it's doable.
The closest I've come to what you describe is my all-Bhaalspawn party:
PC Sorcerer
Imoen
Sarevok
Only one fighter (granted, the best one in the game). It was doable on Core, although I did use many, many Planetars.
It should be possible to do with the party you suggest - Edwin, Kelsey, Tashia and the PC Mage means a whole lot of nasty level 8/9 spells. However, you'll probably what to use quite a few summons. Also remember that you get Balthazar or Bodhi for free if you play your cards right so you will have one dependable creature capable of striking with +4 enchantment (I warn you now, Planetars only strike as +3, so creatures immune to +3 weapons will not be hurt by them).
[!--EDIT|Althernai|Oct 2 2002, 03:44 PM--]