Sincerity is correct
The term implying emotional sincerity is correctly spelled (heartfelt).
You have spelled it correctly, heartfelt, which refers to deep sincerity of feelings.
Yes it 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.
yes, that is correct.
It should be foreign.
No, "infrachrondal" is not spelled correctly. The correct spelling is "infrachondral."