"rapt light being quenched in her eyes..."
See in text (Chapter II - Matthew Cuthbert is surprised)
The phrase "rapt light being quenched in her eyes" expresses the wonder and excitement in her eyes which is stifled or held back.
"stepped lamely into the breach..."
See in text (Chapter III - Marilla Cuthbert is Surprised)
The phrase "stepped lamely into the breach" means to take control of the situation but in an ineffective way.
"the mantle of oblivion..."
See in text (Chapter XVIII - Anne to the Rescue)
Figuratively, a "mantle of oblivion" is a shroud of obscurity. Anne means that she will forget the whole thing ever happened.