It's kindergartner, not kindergartener.
Yes it is spelled correctly
The current culture has adopted the word 'kindergartner' without the extra E you would use in 'Kindergarten'. Webster's dictionary supports this, while the Free Dictionary says you can use either interchangeably.
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.
yes, that is correct.
It should be foreign.
No, "infrachrondal" is not spelled correctly. The correct spelling is "infrachondral."
Correctly: How did Elise spell? She spelled correctly.