Personally I feel anyone manipulating others with a lack of remorse while showing the other listed traits is a severe detriment to society. Grandiosity and self absorption can have other origins and also aren't inherently anti society in the way that manipulating and attacking others without remorse is. For reference I was severely attacked on twitch by an NPD streamer. I have familial ALS and am autistic and wanted to let people know about my condition and all the issues involved. This NPD/psychopathic actually made a fake channel about me, used video of my sleep disorder to make up some sex stuff about me, made up stuff about my family, history etc. They clipped audio from my videos to make audio clips of me saying things etc. Again, I'm terminally ill and autistic. That's the types of things these people do.

Are there objective ways to measure narcissism?
The current diagnostic criteria of narcissistic personality disorder (NPD) requires five out of nine indicated signs (citing DSM-5; APA, 2013):

>1. Has a grandiose sense of self-importance
2. Is preoccupied with fantasies of unlimited success, power, brilliance, beauty, or ideal love
3. Believes s/he is special and unique and can only be understood/associate with other special or high status people/institutions
4. Requires excessive admiration
5. Has a sense of entitlement
6. interpersonally exploitative
7. Lacks empathy: is unwilling to recognize or identify with the feelings/needs of others
8. Is often envious of others and believes others are envious of him/her
9. Shows arrogant, haughty behavior/attitudes

To objectively measure narcissism, there are psychometric tests such as the Narcissistic Personality Scale for this matter. However, research advances challenge the lack of empathy in narcissistic people: stating that an identifying feature of NPD (lack of empathy) is probably misuderstood. So in essence, the tipping point that separates what is normal from what is narcissistic is being shaken again.

Is it when it becomes a problem for others? For ourselves?

This is generally correct, depending on the perspective. A clinically diagnosable disorder is generally defined as a condition that "causes clinically significant impairment in social, occupational, or other important areas of current functioning.(APA, 2013)" You can see that diagnostic criteria in most mental health disorders like mood disorders (depression), anxiety disorders, trauma-related disorders, etc.

However, this criteria isn't present in personality disorders. The unifying feature of personality disorders is that it has specific pervasive patterns of behavior that begins by early adulthood. That's it. I am convinced to think that your question is still under debate of researchers worldwide.

However, in practice, clients seek the diagnosis for personality disorders when it causes significant distress on them or on other people. So it becomes NPD (or other PD) when it becomes problematic.

I think the line is remorse. Upon self-examination, healthy individuals will feel regret for the people they’ve hurt. In contrast, the ego of NPDs will not allow this. Just my hypothesis

I believe that all psychological traits/disorders are on a scale ( eg autism). And they have set criteria for when they graduate from trait to disorder. The set criteria are defined by professional consensus and are again on a sliding scale where not all are required and each will present at different severity.

When the manipulation is causing harm to you or others.

There is nothing wrong with manipulating yourself or others to be more productive, to engage in win win, mutually beneficial outcomes, and so on. It becomes pathological when you are using manipulation to essentially steal from others, engaging them in scenarios where you disproportionately benefit at their cost, or/and where you harm yourself.