Yes, "negotiable" is spelled correctly.
You have spelled 'still' correctly.
Yes, engineer is spelled correctly in your question.
Flattest is spelled correctly. The others are spelled: silliest, biggest and sweetest.
Yes, surprisingly is spelled with an S. Surprisingly is how you spell it. You spelled it correctly.
You've spelled it correctly.
Convenience (easy access or availability) is correctly spelled.
Correctly: How did Elise spell? She spelled correctly.
it is correctly spelled iPod.
It's kindergartner, not kindergartener.
You spelled it correctly: pumpkin.
You've spelled it correctly.