Does the failure to complete an engagement after performing the Istikhara prayer indicate a lack of sincerity on the part of the worshiper, or does Istikhara not prevent an event from occurring and then not being completed?

Istikhara is a request to God for good in one's affairs. The failure to complete a marriage after performing Istikhara does not mean a lack of sincerity in it. Rather, it might be what is best for you, for what God chooses is good, even if it does not align with your desires. This is in accordance with His, the Almighty's, saying: "But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah Knows, while you know not."