Uller, let me get this straight...
You don't like the cost of a feat (ie Still spell) to overcome the armor problem, but you think there should be cost to pay for the ability to cast in armor.
At the end of your first post you mention feats.
You would rather have your feats instead of their feats?
I see about four house rules you could use here.
1, Offer a costly (so they can't get it until level 5 or so)masterwork armor that provides little protection, say +2, but does not cover any of the major joints on the casters body so it will not interfer with casting. So there is a very small breast plate, maybe something on the back crafted to flex with the backbone, some armor pieces on the upper arms and legs. If WOTC can fabricate two handed weapons and repeating crossbows, then this is not that far off.
Second, give sorcerers the ability to cast spells of the next higher spell level one character level before they know any. In other words, at character level 5 he gets to cast 1 third level spell (just like a wizard) but he does not know any until character level 6. This does two things, first he can cast stilled second level spells in that slot, and also allows him to create magice items at the lowest (and cheapest) effective level.
C, offer another feat (JUICE) that reduces the effective level of a metamagic spell by one. So a Juiced Maximized spell still goes off at +2 levels, but the juiced stilled spell goes off at +0 levels.
Finally, allow the user to hold a shield but not weild it as a free action, so if he is far from the action, Thangor is holding his shield but not using it for defense, when the enemy engages, he starts actively defending with the shield and casting stilled spells.