I'll address some of the things you said and then link two keeper builds with a lot of information, some of which may answer questions you have about keeper.
Level 20 keeper: Because you have two of the best staves in the game, you should be able to do threatening runs well as a level 2+ keeper, so there is really no need to stay as a level 20 when doing it. Playing as a level 20 even in perilous is uncommon, and definitely unnecessary for a keeper. Keeper is a great class for low promotion players because their key abilities have components not dependent upon attributes.
Barrier: Whatever you do, and no matter how confusing things get, always cast barrier on yourself, even 5-20 seconds before a fight (to benefit from increase mana regeneration and/or mana surge). Time recasting so that a few seconds before barrier becomes available, you are close to at least one other teammate, so that you can barrier two or more people at once for more barrier cooldown from the Elegant Defense upgrade.
Frost step: This is decent on the keeper, but a few disadvantages to keep in mind. 1. It breaks winter stillness, which is a very good passive if you can stand still a lot. 2. You usually don't want to be focusing on damaging enemies by running through them, you want to focus on where teammates are and standing near them to barrier them. 3. Fade step (no frost step upgrade) will allow you to be speedy, and optimize use of winter stillness passive by standing still longer and then sprinting with fade step to get close to teammates to barrier, and imo can be better than getting the frost step upgrade as the keeper shouldn't be focusing on DPS.
Static cage: This does double the stated weapon damage when enemies within it are struck by basic attacks because the runes are counted separate. The enhanced static cage ring increases duration by 70% instead of 30% and should always be slotted on a keeper if you have it.
Chain lightning: The damage is good, the cooldown is fast, it staggers enemies, it reveals stealthy enemies, and it can proc static cage damage and clean burn many times. This is where you dump excess mana when barrier and static cage are in cooldown. It may be boring to you, but keep in mind, if you decide to replace it with something, you will have a wasted ability point. IMO, barrage/lightning bolt are very very bad options vs. chain lightning.
http://forum.bioware...vable-laziness/
http://forum.bioware...r-with-barrier/