Valeria and Ezren are at the same location. Ezren Explores and turns up a low level monster. Ezren has only one spell in hand. Valeros has a few weapons.
It seems that Ezren and only Ezren must attempt to defeat the monster. Valeros can add to Ezren's check in a number of ways but he cannot step up and stab the monster (make a melee check in place of Ezren.
Am I reading this correctly?
Valeros could make a check only if the Bane card listed a two conditions with Then linking them.
Similarly, Valeros Explores and turns up some Arcane boon. Can Ezren make the check to acquire this boon?
The character that encountered the boon or bane is the one that attempts the check, unless there is a then connecting 2 checks, in that case the active character can let another character attempt one of the checks..
Correct, the Activated player, the one who explored for example, usually gets to pick the attack type or skill check used. Then, other players may "assist" (buff) him, but not with another weapon or attack spell, they may only assist with things like blessings, buffing spells, or allies that would add to the same type of attack/skill the activated player chose to use.
Make sure to look for the specific text "Your check", "A check" (which usually can be used on anyone, anywhere), and "At your location" when seeing if a card can help a friend with a check.