Two suggestions.

1. When you read the ini file, you can improve retrieval significantly.
a. Your lines can be keyed when adding to the collection: Selection.KeyName
b. Now your ReadString can quickly return value via the collection key: Selection & "." & KeyName; no looping needed

2. Most people that read INIs also need to update them too. Maybe consider adding functions to your class to change the collection items & write collection to an INI file.

As long as you keep your ini sections in order, within the collection, you shouldn't have any problems with any of your other functions.