Your university advisor was right, the IO and your 'visa consultant' are wrong.
You shouldn't face a ban if you overstayed for less than 28 days unless you have also done something else which might trigger a ban. See
320 7B
You should declare the overstay since they are already aware of it, and if you don't, you are correct that you will then risk a 10 year ban. Be honest and you should be OK. Consult with your university student advisor again and ignore the visa consultant he's clearly not worth the money you are paying him.