OK, I whipped this up quickly. Take a look and see if you have any issues or questions. I reverted the Amounts column back to a user specific column and re-enabled row owners, so any user data is protected except for what is added to the leaderboard table. There is use of Custom actions when viewing the activity details, and on the buttons to submit or cancel the point entry. Pay specific attention to how I set up those actions.
Instead of using the native form, I’m using a custom form based on the activity details screen. (It’s just entry components and buttons with actions). When a user enters points, it’s saved to a temporary column. If they cancel out of the form, only the temporary value is cleared. If they submit, then the temporary Amounts value is written to the Submitted Amounts value. Also, the submit will check if the user’s rowid is already in the leaderboard table. If it is not, then it will add a new row with the total accumulated points from all activities, along with their name and image. If they already have a row in the leaderboard, then it will simply update that existing record through the relation.
Copy it, play with it, and make it better. 