Just create 3 different groups - 1 for each property - and then create the categories for each group. When a transaction comes in that needs to divided between the properties, simply use the Split tool in Tiller and split as needed, assigning to the applicable categories. You could use Autocat to help automate some of this, but some work would be needed on your end to split the transactions accordingly.
Groups: 9219, Cham, Mortgage
Categories: 9219 Software, 9219 Paper Supplies, etc (make sure to assign the Parent Group for each on the Categories sheet).
For the IRS, you could use something like the Transaction Tracker which allows you to filter pretty granularly to get the total Group transactions (Property) for a specific time period. I think this is what you need. It shows the Category in the output, the amount, Date of transaction, etc, so I think they could use this.
In terms of previous Categories you use, you can just create the new Group / Category structure I mentioned and then go back and rename the old categories to what you want them to be so they fit in with your new Group / Category naming structure. Double check your data to make sure what you’re adjusting doesn’t have transactions which are for more than one property (i.e. not Split) and assign to the new categories as needed.
That’s how I would do it, if I were using Tiller to manage this.