It is permissible to use the miswak or the like when fasting, but one should avoid swallowing the liquids from outside the mouth. For example, when one uses the miswak and it becomes wet from saliva, if it is taken out of the mouth and placed back in the mouth again, the wetness on the miswak will be deemed to be from outside the mouth, and swallowing such liquid will invalidate the fast.
The time in the said marriage is the right of the husband and only he can forgo it. But it is permissible for the wife to make a condition in the contract that she acts on his behalf in this matter. She can then forgo the remaining time.