Do the monsters engage and attack directly Summoned stones (they are not figures, only obstacles!)?

If not: if players close a corridor with summoned stones, do the monsters get blocked for ever?

The summoned stones are familiars treated as obstacles, but their card text states they can be attacked. As such, RTL actions that require monsters to attack can target them instead of the heroes.

From the rule-book

Familiars treated as figures are not affected by quest rules or perils. When a monster is instructed to target a hero, tokens that can be targeted by attacks and figures treated as heroes are included as possible targets. For the purpose of targeting, if a familiar or attackable token does not have the statistic the monster is targeting, that familiar or token is considered to have a value of 0 for the corresponding statistic.

I overread this as well, but basically the app should say "target" instead of "hero" imo.