Wrong. The colonies on the ampicillin plates are a result of contamination of the original vector from which the PCR product was obtained.

Possible - The larger the fragment is, the longer incubation time is required. Use over-night incubation with BP.

Wrong It is unlikely that this will result in few colonies. Most likely this will give us different products inside the pENTER vector.

Very likely, free primers that will participate in the BP reaction will compete for the same recombination sites but will result in abortive vector.

Possible. If all other options were checked and rooled out, design new primers and start all over again.