I don't think saying "RAW" is a good way to look at it; RAW is underspecified, that's the issue, and we might as well interpret it in the way that best fits RAI.
Personally, I'd let Widen Spell work on Cluster Bomb or Concentrated Fire, neither of those pairs seem OP alone, but I wouldn't let those two tricks be used on the same spell.
Mist Screen (Heighten Spell): You can shape your obscuring mist into a dense wall of mist with an area up to one 10-foot square per 2 levels. The sections of wall are 5 feet thick and must remain contiguous, but they do not need to be anchored to a solid surface. At least one section of the wall must include your space. Creatures standing within the wall have concealment. Creatures on the opposite side of the wall have total concealment.
Obscure Self (Stealth 6 ranks): You reduce the area of the obscuring mist to your space plus all adjacent squares, but the mist is no longer stationary and instead automatically moves with you when you move. This does not create enough concealment to attempt Stealth checks. As a swift action, you can anchor the mist, causing it to become stationary and allowing enough concealment to hide. You can release the mist as a swift action, allowing it to move with you again. If you take any fire damage while this trick is in effect, the spell immediately ends.
Looking at the Obscuring Mist tricks, did the writers intend for you to be able to make a giant wall of mist that also moves when you move? No, I don't think so. And from there we can infer the general principle that you can use at most one trick on a spell.