Hello to everyone, who's willing to answer to my questions.
First of all, I have a problem with superpositions of definition of SWIFT action and difinition of using magical item BELT OF BATTLE (Magic Item Compendium). It's unclear to me whether the item gives opportunity to use second swift during the round it has been used or not.
Second is almost the same problem with superposition of SWIFT action and POWER "dual actions" of prestige class "thrall of Demogorgon" (Book of Vile Darkness). Whether am I allowed to use two swift actions during the same round I've used the power or not?
The third is the most complicated - what is happening when I activate the BELT OF BATTLE and power "DUAL ACTIONS" in the same round? How many actions and what type I supposed to use?
One more here to say is about spells and attacks. I'm allowed to cast just ONE quickened spell per round. Is it possible to cast extra quickened spell in round because of using described powers and items, which grants you additional swift actions? If not I see some kind of imbalance for casters in comparisons with warriors, because caster could use standard for ONE spell only and no quickened, and warriors are allowed to do several full-round attacks for the same amount of time.
Then the forth is not clear question also. Am I allowed to change standart or move for swift action??? From definition of SWIFT we could see that it's needed far more less time to do swift than move and standart actions. So logically we are coming to possibility of using three swift actions per round exchanged from standart and move...
p.s. sorry for possible mistakes