So not only she can summon it, but also decide where it appears?
Obviously. The Beast is called to the village to punish it, yet waits until Abigail is in the hazard to wipe the entire village clean because... why exactly? Because Abigail is the straw that breaks the camels back? Ridiculous considering the other far more heinous things the village had done by that point. "I'm brewing up a spell that is mighty enough for the whole village" - Whole village is wiped out a few minutes later - "Gee, I wonder what happened? It is a mystery." She kicked the bee's nest.
A Witch is not a Sorceress in the Witcher lore. There's a big difference in their capabilities. Yes, she was a cultist of the lionhead spider, making her perfectly capable of doing dark shit. If she had the capability to perform on the level of a proper magician, she would be advising kings, not curing warts. There was no reason for her to stick around in that village that she openly admits hating unless she was there to see vengeance.
The Lady of the Lake is wrong, it's not the first time she was in error either. Whatever answer you choose, the Beast is defeated, meaning both are guilty. Ignore the npcs in the game trying to guilt-trip you over "innocent" wahmen. Abigail is not innocent. Selling poison to a suicidal girl is enough to tell you this even if nothing else were true. The villagers just happen to be worse.