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Dailies are always the highest sale they have in a Steam sale event.
Popular dailies may be extended to two days or get into the encore last day pack. If that exists.

As a side note: For non-indie games, Steam is usually far from being the cheapest option to buy games for yourself. Authorised resellers often have better deals. For example, the best deals Steam now has on the two modern Fallouts (FO3 and New Vegas) are 75% on the base game and 66% on the complete editions. GamersGate had 75-85% sales on the same games more than once in the past year alone. Using IsThereAnyDeal is suggested if you are tight on budget and looking for something that is not bound to get even in a Humble Bundle any time soon (like, to use that example again, any Bethesda-published game– if you can bet on what AAA publisher will never have a HB, it's them).

Yes. But if you don't play the game for over 2 hours you can always request a refund and buy it again at a lower price. However it can take a week or more to get your money back.

Depends on the scale of the sale. IIRC last two Halloween sales didn't have any dailies or anything, just one big page of deals that all ran for the whole duration of the event.