Hi,
From what I have done "fored" planning level in my previous project (assigning the different planning level for Bills of exchange presented to banks), the change in BSEG was reflected correctly in FDSB table (summary table for cash position) and the drilldown to FBL3N also shows all the items belonging to the planning level correctly. You can doublecheck if you change the planning level by FB02, the system updates the FDSB table on the background and you can verify that.
10 account range would give you an additional benefit if the cash position analysis business wants to do is as per House banks, however, if not, it would not be absolutely required to achieve what you described as the requirement.
Cheers
taro