Hi Rise and shine,
If I'm to understand your situation correctly, your mother is depositing the funds from an FD, that was in her name, to your savings account, and this will account for 75% of your funds being shown as proof?
If that's the situation and, assuming you get an ITA soon, I believe you will need to get a Gift Deed to show that the funds from your mother are being gifted to you and will not have to be returned, so you are free and clear to use them for settlement purposes. If CIC sees that large single deposit to your account in the letters and statements that you'll need to provide as proof, they're going to want to know where it came from, and you need to provide evidence that it was gifted to you with no conditions attached. Otherwise they may be suspicious that you got a loan from someone to try to get to the threshold (some people apparently try to do that) which isn't allowed...