Thank you for your comment.

1. User B cannot post a Memo on User A's account. It would not be feasible for the app to check every participant's account for their Memo settings. The purpose of this system is to have a unified Smart Contract Account that contains all the variables, so that any user can alter any variable by making a payment to the Smart Contract Account.
2. I would not make any money from this, and there would be no hosting expense because the app would be entirely run client-side. I would post it on Github, since it appears that this subreddit doesn't allow zip file downloads.
3. By combining the values from multiple payment types, each final variable can contain more than 4 digits. For example, Payments for .0001 - .0001999 AND .0002 - .0002999 would allow values of 0 to 99,999,999.