You can use the "Transfer Division of Interest" Utility under the Utilities menu to do this. Choose the Dummy owner as the original owner at top, then fill in each royalty owner in the fields at the bottom with the percentage of the 12.5% that they should receive. Mark the option to include owner and suspense history and choose the run that first created suspense for this dummy royalty owner. This will transfer all of the interest and suspense from the dummy owner to the new owners.
Create a backup before you do this so that you have it on file if needed.
You had the right idea when you set this up, but I do not recommend putting the word "Dummy" in the unknown owner's name. The word Dummy triggers the software to gross up the well numbers which may not have been the desired result. This should still work out fine for you even though you used dummy in the name but if you do this again in the future just replace the word "Dummy" with "Unknown" or any other word you like.
Once a check has been sent out, there's not a way to re-coup that money. About the only way to get it back is to make a negative revenue entry for that well on the Well Revenue by Well screen, and fill in that owner in the "Allocate All To" field. This will give them a negative income amount, which will then be put against any future earnings that they'll have.
You can also enter a suspense balance for that owner. The "Allocate All To" method is a little simpler, but you're welcome to enter a suspense balance, too.
You'd need to go to the Maintain > Suspense History screen, and add a new record. Fill in the owner, well, group, and the run number and run year of the last closing for that well. Next, enter the amount of revenue, taxes, and the net amount. Set the action as Into Suspense and the type as Minimum, and then click Save. That will create a balance for them, which will be collected, and taken out of their future earnings. Enter the revenue, taxes, and net amount all as positive numbers.
Make sure when printing the Owner Distribution Statements that you have the option checked to Print Minimum Amount Statements.
Return to Suspense