1

I have a two office client connected with a 10 MB fiber point-to-point network.

4 Spice ups
2

Yes, but Remote Collectors are recommended.

As long as Spiceworks can get to the network (routes exist), it can scan it.

3

As long as SW has a route to use between the LANs, yes. I inventory 4 subnets over P2P VPN tunnels with no remote data collectors.

4

I have several subnets. You can have Spiceworks scan several subnets in the scan settings. It should be fine with just two systems on the remote vlan, but if you start adding more machines you will want a remote collector like Lee said, especially over a slower connection.