Currently, a Daggerspell mage caster build pretty much needs the full set to be viable versus a non-set or Arcane set caster for 3 reasons:
1) Lack of shield AC without set, meaning a non set Daggerspell will have cripplingly low AC.
2) Difficulty in getting Uncanny dodge without losing caster levels. The set ring provides this essential feat. Not using the set means a single level Cleric or a 2 level investment in SD/Scout/Defender or Barbarian if you want to keep max caster level (which is important). Non DSM arcanes have a lot more flexibility in this regard, as DSM locks down 3/4 classes (Arcane base + sneak base + DSM)
3) Feat starvation. Taking 10 levels of DSM means losing up to 3 Epic Feats, and requires taking useless Weapon Focus and TWF feats. This is alleviated by metamagic (Emp, Maximize) on the set.
1) To make the lack of a set less crippling for a DSM AC-wise, perhaps the level 1 Arcane 'Shield' spell can be modified to grant 4 + CL/6 Shield Bonus AC instead. Alternatively (because Warlock DSMs are technically possible), Shield AC = 2 + DSM Class level can be instituted as a class feature. The shield AC can then be moved away from the set.
To compensate for the weakening of the set's overall power, perhaps a spell equivalent of the Rogue's Epic Precision could be implemented (1/2 sneak dmg vs immune), as DSMs do weak damage vs Sneak/Crit immunes on Live.
2) Add Uncanny Dodge as a class feature at DSM 2. This is a non-spell progression level, so it makes sense to add some features here. The Uncanny Dodge on the set ring can be replaced with Weapon Focus or Improved Critical on Ranged Touch Attacks.
3) I don't think this needs fixing, as DSM does grant some advantages, and some feats can be found on other items. A significant feat cost is fair.
These changes should make a non-set DSM viable, but having the set is still a significant bonus.