For BAB you can always use Arcane Disciple to snag Divine Power. That is in fact probably your only option for the 4th iterative that doesn't involve early entry cheese for Shadowcraft Mage, since it'll lose you three points of BAB if taken to completion and Beguiler loses you one at 1st level and one at 3rd unless you're playing with fractional BAB rules (in which case you can take a whole 3 levels of Beguiler and still conceivably reach your 4th iterative). I don't know of any gnome deities who offer domains with Divine Power offhand though.

You could alternately use something like Cleric 4/Ordained Champion 5/Shadowcraft Mage 5/Full BAB with as much casting as possible 6. Might not get you to 9ths (indeed it probably won't), but you said you're prioritizing BAB for a low-power game. A higher power version might be something like Duskblade 3/Sneak Attack Thug Fighter 1/Full BAB 1/Ur-Priest 4/Shadowcraft Mage 5/Full BAB 6.

Or, since you want to be unarmored and pretty focused in terms of casting, you could use something like Stalwart Battle Sorcerer 4/Battle Dancer 1/Spellsword 1/Abjurant Champ 3/Shadowcraft Mage 5/Abjurant Champ +2/Eldritch Knight 4.

None of these have much sneak attack, besides possibly Martial Study (Assassin's Stance), though the Ur-Priest does have 1 dice as a class feature, which is enough for Craven.

For more sneak attack there's always Unseen Seer. Even a few levels for Hunter's Eye through Advanced Learning might be a good idea, but it basically forces you into the Arcane Disciple->Divine Power route if you want a 4th iterative, and it sounds like you do.

I suppose you could always just set a few caster levels on fire and go with something to the effect of Beguiler 2/Sneak Attack Fighter 3/Spellsword 1/Abjurant Champ 5/Shadowcraft Mage 5/Full BAB 4.