Just to comment on the statuses and how we discover computers - internally the Site Manager uses the standard Windows API for getting a list of computers - this should give the same list of computers as the network view of explorer or the 'net view' commmand line command. We've observed that sometimes this system can have problems as it relies on master browser negotiation.
If your Site Manager server doesn't list the computers which are listed by 'net view' or are listed in explorer (where the 'discovery method' column in the details view is 'NetBIOS'. Computers where the discovery method is 'WSD' use a different system), then this may be a Site Manager bug.
As far as the computer status being offline, this is because if there is no agent installed, we list computers as 'No Agent' if Windows networking lists the computer as present and 'Offline' if not, so I suspect your computers are listed as offline because of the same issue which causes the Windows cimputer list to be empty.
You should be able to just install the agent using the Remote Install functionality (or manually install it) regardless of the computer being in the offline state.