of determining which Act II quest is available, any Act I quest that the heroes did not attempt is assumed to have ended in an overlord victory." - so OL always win at least 2 quests? Why?>
There are a total of 10 Act II quests in the campaign, two for each Act I quest. These Act II quests are linked to their respective Act I quest both in theme and in purpose. In some cases, there are relics which must belong to either the heroes or the OL at the beginning of the Act II quest(s) involved and who has it is determined by who won the Act I quest.
Therefore, it is important that each Act I quest has a "winner" among heroes and OL, so that the approriate Act II quest can be played as it should be. The rule about the OL winning any Act I quests that weren't attempted is just to provide a definite answer to the question of "who won a quest we didn't play?"
Note that the total number of quests won by each side has absoltuely no bearing on the outcome of the campaign. The side that wins the finale wins it all, even if the finale is the only quest they actually did win in the entire campaign. The OL doesn't gain any mechanical advantage by having the two unplayed Act I quests "awarded" to him after the interlude.
E) In the Act 2 they must play The monster's hoard, The twin idols and The wyrm turns in that order.
No; whoever wins the interlude decides which of the 5 available Act II quests will get played next. Whoever wins that quest picks the enxt one, etc. The same way as in Act I. Which of the two Act II quests is available for each set depends on who won the linked Act I quest. The OL is considered to have won the two quests that weren't played for determining which quest is available in Act II.
2. Do OL Lieutenants count as monsters? When OL/Heroes card/skill says "monster", it means also Lieutenant? Can Lieutenant attack twice, or only once like monsters?
Yes, LTs are monsters and may be targeted by any effect that says "monster." Unless the effect explicitly excludes Lieutenants, of course.
If OL kills Reanimate, he doesn't gain card like for killing Hero, and Necromancer call the Reanimate for no cost (except the action) again his round.
Summoning the Reanimate costs 1 fatigue as well as an action (that's what the the fatigue symbol in the lower right corner of the card means.) Also, as Robin mentions, an action is no small cost to pay for any effect in this game.
4. If the monster/Lieutenant has ability, which is attack (Lord Farrow - Ignite) does it count like attack, so he must choose - attack normally or the special attack, or is it the ability, so doesn't count as attack?
It counts as an attack if the text of the ability says "make an attack." (Ignite is indeed an attack.) Yes, if the monster uses an ability that makes an attack, that counts as its one attack for the turn.
Most attack abilities will require an action to use, however, not all abilities that require an action are necessarily attacks.
5. Items from loot (potions) are one-time use for quest, but can be selled even if used?
Yes, search cards may be sold even if they were consumed during the quest. In fact, search cards must be sold at the end of the quest, whether they were used or not. Otherwise the Search deck would quickly be reduced to just the Treasure Chest and the Nothing cards and the heroes would be laughing their way past the Searching element of the game.
I like to think of it as a "potion bottle recycling" policy. Merchants will pay good coin for empty potion bottles and reuse them later, like beer bottles in the real world (only you get a sizable amount of coin for potion bottles. =P)