Based on what u explained above, u are exploring a "possible hack" so u can indirectly withdraw your CPF OA prior to 55.
For u to confirm if this hack works entirely and properly, it is best to meet HDB to discuss the conditions and requirements u need to meet before this transfer can be executed as u planned (without telling them the last bit where your mum will give u money

)
Since u want to get the cash out ultimately, u would be exploring a resale transaction (not just add u as additional owner or transfer ownership), read some info here
https://www.hdb.gov.sg/cs/infoweb/r...changing-owners-occupiers/change-in-ownership
Per the hdb website which talks about part sale, it can be done between family members at a mutually agreed price. What about full sale?
You will have to pay your mum at least the cpf monies and accrued interest to be returned back to her CPF. If she is eligible to withdraw cos she is above 55, she can withdraw after that.
As to the $350k resale price, u have to check/confirm how much u are eligible to withdraw from your CPF for housing. Confirm with HDB whether independent valuation is required to withdraw that amt from CPF. If everything is ok, u can use your CPF of $350k to pay your mum, part CPF in cash and part into her CPF.
There u are your hack! Trust u will take good care of your mum
(i have no experience, just logical thinking based on what I read at HDB website)