Yes! El Salvador uses the same plugs as the US so you shouldn't have any issues or need adapters.
Yes! There will be shuttles to and from all wedding events from the Hilton San Salvador. Details about timing for all shuttles can be found in the "Schedule" section of the website.
There are a few options for transportation to/from the airport. Please see the "Travel & Accommodations" section of the website for additional details.
El Salvador uses the US Dollar and most places now accept credit cards!
When going through customs in the San Salvador airport, they do charge a $12 per person tourist fee. This is normal for them! It can be paid with a credit card.
In most countries, you can only shop duty free as you leave the country but in El Salvador you can shop Duty Free after you land :)