Would'nt the last basic inventory or the last configuration fields be the most accurate fields to use to indentify in-active machine. The fields and data from the Inv_Inventory_Results table is from the last inventory. Which may or not work depending on the configured Inventory Plug-in policies.
I would think the last basic inventory field [Client Date] from the Inv_Aex_AC_Indentification table and/or the _eventtime from the Evt_NS_client _Config_Request table would be the most accruate in indenity inactive machines.
Here is a query that I use to indentify in-active machines in my environment I also have included the Last inventory collection field.
declare @days integer
set @days = 90
SELECT
VC.Name AS COMPUTER,
VC.[OS Name],
vc.[User] as 'Logon User',
vc.IsManaged,
ISNULL(CAST(ai.[Client Date] AS NVARCHAR), '') as 'Last Basic Inventory',
DATEDIFF(dd,ai.[Client Date],GETDATE())as 'Days since Last BI',
ISNULL(CAST(cr.[Last Config Request] AS NVARCHAR), '') as 'Last Config Request',
DATEDIFF(dd,cr.[Last Config Request],GETDATE())as 'Days since Last CR',
ISNULL(CAST(ir.[Collection Time] AS NVARCHAR), '') as 'Last Inventory Collection',
DATEDIFF(dd,ir.[Collection Time],GETDATE())as 'Days since Last IC'
FROM vComputer VC
left join ( select resourceguid, max(_eventtime) as 'Last Config Request'
From Evt_NS_Client_Config_Request group by resourceguid ) cr
on cr.ResourceGuid = vc.Guid
left join Inv_AeX_AC_Identification ai on ai._ResourceGuid = vc.Guid
left join Inv_Inventory_Results ir on ir._ResourceGuid = vc.Guid
where DATEDIFF(dd,ai.[Client Date],GETDATE()) >= @Days
or DATEDIFF(dd,cr.[Last Config Request],GETDATE()) >= @Days