I purchased some paint. when I applied it to the wall it was too dark, can I bring the paint back for additional white to be added. I know there will be a cost

Yes

Yes you definitely can

Yes...with your receipt.

Depends what shade of white you want did you prime it before painting the shade you painted it ? Sometimes it is better to get a Pro like me to do it right the first time and yes when people try to do it themselves and fail it costs all of us contractors in higher prices and higher level of costs to the consumers that should get a Pro.

I'm pretty sue you can.