Flattest is spelled correctly. The others are spelled: silliest, biggest and sweetest.
flattest
You have spelled 'still' correctly.
Yes, "negotiable" is spelled correctly.
Yes, engineer is spelled correctly in your question.
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've spelled it correctly.