MInor changes in flutter project

The app is like a puzzle/quiz game that the user inputs his answer to the quiz based on an onsceen keyboard and progresses the levels. Every level has a hint system and also a success page after correct answer
I need someone to make the following changes to a flutter code:

a) fix a bug regarding the paid hint. Specs: If I buy the paid hint in level 1 I must see the paid hint up until I pass the specific level. (That works fine). If I pass level 1 and go to level 2 then immediatelly the level 1 hint should stop being visible. Currently level 1 hint is shown in level 2 and level 2 paid hint is unavailable unless if I exit to main menu and come back to the question

b)add an interstitial ad before success page page

c)create a test screen (using the existing code) for validating each riddle ( I will have each time to enter the doc name from firebase)

d) check if loading time could be reduced with a quick and dirty way