How do I ensure that Salatul-Istikhara was performed correctly, and am I to blame if I make a wrong decision after performing it?

As long as you have sought guidance from Allah Almighty (istikhara), He will choose for you what is most righteous and beneficial. Therefore, you should not hesitate after that. There is no harm in consulting a trustworthy and sincere advisor, for "he who seeks guidance (istikhara) and consults (istishara) will not regret."