Are local GOMARs and Article 15's a career ender for Officers?

I would have to say it depends on the infraction, will it be applied to your OER, and how much scrutiny will be placed upon you by the Chain of Command. A local reprimand or Art. 15 will not follow you, what will follow you is your OER's, and people you serve with. An awful lot depends upon how the officer in question rebounds from this. If the officer takes the self pity route, or the outraged route, it could have negative repercussions, if instead they take the setback in stride, acknowledge mistakes were made, learns from it, and doesn't make the same mistake again, I see no reason why it would end a career. The Army usually attempts to rehabilitate SM's that make small mistakes.

No if it is filed locally as opposed to officially.

If I remember locals don't because it doesn't follow you ( especially when you PCS) as its not part of your OMPF.

As far as an article 15. I think this is a good thread to read:
https://www.rallypoint.com/answers/in-today-s-military-can-you-survive-an-article-15-or-letter-of-reprimand-and-still-have-a-successful-career