Yes, that works for me, but you should use "was" rather than "is". You are talking about something that has happened, not something that is happening.
However, feckless is not a word that I have used that often. I would probably use a more popular synonym such as:
His irresponsible behavior / lack of planning / ineffectiveness / reckless decisions/ , made our company bankrupt.
It was the final nail in the coffin.
Another synomym for final nail in the coffin is:
It was the straw that broke the camel's back!
Rather than saying that he was the final nail in the coffin, I would tend to say "It was the final nail in the coffin."
Your existing comment works and would also be understood though :-)