from my understanding it is a safeguard to preserve life and for cases where people are lacking mental capacity
It isn't anything to do with preserving life.
It's only for people who lack capacity to continue to consent to their placement in a care home or hospital because in this country we can't detain people of their liberty without their consent or without a legal process.
People who can consent to being placed there don't need a DoLs, people who can't consent to being placed there do need a DoLs. The DoLs is part of the Mental Capacity Act.
There is a very clear process to follow if the person or their relatives object to the DoLs being put in place which will be explained to you as part of the process if it even gets that far.
The fact that someone is doing an assessment doesn't mean she will automatically put under a DoLs, the assessment is fulfilling the local authority's legal duty to determine if she needs to have her placement authorised as a DoL.
There are different legal process for people who can consent to being detained, but don't consent, but still need to be detained against their will (e.g in prison or hospital) and these are through the criminal courts and the mental health act.
Their only agenda will be to determine if she requires a DoLs authorisation or not.