You have a lot of questions, I have tried to answer them all - Please raise a hand if you are still left with a question.
ACSLS is able to control a large amount of STK silos but luckily Netbackup has a great integration with ACSLS . When performing a Netbackup robot inventory, ACSLS return the location (ACS,LSM,panel,row,column) of the tapes along with it type (HCART,9840,LTO etc etc). This way Netbackup (and ACSLS) ensures that tapes are not mounted across non' PTP'ed robots and a 9840 tapes isn't inserted into a LTO tape drive.
Only robot that are PTP'ed will appear as one logical unit (the ACS will be the same but you will have multiple LSM's just like today). cmd_proc is a ACSLS tool and can always see all robots,tapes and drives, but that does not mean the mapping in Netbackup is the same. Query from Netbackup also return info from both robot because ACSSL control them both.
You don't need inventorying filter if it's the same backup system that access both SL8500. If it's different system, use ACSLS volume access control.