My first piece of advice, Rory, is not to worry too much about making a "terrible mistake" - because it isn't!
Currently, you do not have sources close to Matilda's birth to determine her correct last name at birth/LNAB. Until you find birth-related sources, you should leave her current LNAB as is and add the name from her daughter's death certificate to the Other Last Name field(s).
At WikiTree, we like to avoid unnecessary LNAB changes because each change creates a redirect; over time, the accumulated redirects increase costs and cause server strain (we want to avoid those as much as possible!). If you change a LNAB now, only to find it is also incorrect, then you will need to change it again (again, increased costs, server strain, general inefficiency).
So, until you can PROVE the LNAB (preferably with birth-related sources), you should not make change it.