I would like to set up ICP reports by inserting the plug account only.
I have a staged consolidation. Do I need to create a separate tree where all the entities are included in a flat way?
There is no reason for a seperate tree.
The eliminations take place at first common parent. So, if you create a report that check one entity against the others, the system should produce the correct differences. Since you are yusing the plug account, pay attention to the account type attribute of each account.