The mortgage approval may not really have anything to do with it. Once Mom has given up "control and dominion" (see Treas. Reg. 25.2511-2 https://www.law.cornell.edu/cfr/text/26/25.2511-2) the gift is complete. Depositing the money is the account seems like giving up control. Unless there is a legally enforceable agreement to the contrary. Wouldn't want to litigate that one.
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