It is always very difficult to answer hypothetical situations of this nature. However, looking at the clues in the text, it becomes clear that Mrs. Twycott would have waited until her son had died before making any moves to marry again. Note how this is indicated when she is quite happy to go along with her son's wishes overtly, even going as far as to swear that she will not marry Samuel Hobson:
The poor woman swore, thinking he would soften as soon as he was ordained and in full swing of clerical work. But he did not. His education had by this time sufficiently ousted his humanity to keep him quite firm; though his mother might have led an idyllic life with her faithful fruiterer and greengrocer, and nobody have been anything the worse in the world.
Mrs. Twycott's policy is always to give in to her son's wishes, no matter how much she is inconvenienced by doing so, and even if she is not able to live the kind of life she would like to live. The quote above clearly indicates that if Mrs. Twycott knew that her son were to die for any reason, she would obviously do nothing to confront him or go against his wishes whilst he was alive, but would wait until he had died and she would not have to confront him or challenge him openly in order to marry her childhood sweetheart. The way the text describes her son's power and hold over her makes any thought of her defying him, whilst living, impossible.
sorry it's one, not once.