I have been thinking about your notes idea for a while. I wasn’t really sure how I felt about it at first. On one hand a user could enter all the grammar notes they want themselves but on the other hand it seems like a lot of extra work for the user. What if you divided the notes section like the info section between the INFO and STATS panel. You could have a GRAMMAR panel and users notes panel. For sentences created by Skritter the GRAMMAR panel would be fixed like word definitions. Basically everyone would see the same grammar definition for a sentence. But the user notes would be more like mnemonics where users could share and see other users notes. For grammar on sentences created by users you could have the grammar split into different categories like in the Bunpo app.
For example the sentence creator would select if it is a
Ask for Permission
If they select Comparison a list of the different ways to make a comparison could be provided. This would make adding grammar notes easier. If the sentence grammar is wrong the user could then use the same method to select the correct grammar and a message would be sent to the creator of the sentence asking if they want to change the current default grammar definition to the one changed by the user. That way other users would also then see the correct grammar definition.
If you do it this way that would also mean that your sentences could be grouped by grammar. Maybe later on you could offer study options like review all words with N4 Suggestion sentence Grammar. I feel this could be a big help to students who want to review grammar.