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The MAC comes with screen sharing software built in. I currently rely on that to do what you are wanting.
I turn on screen sharing on the MAC (not in front of a MAC right now to tell you exactly where to go in settings), and connect with it through the free software client VNCViewer. That way you use your existing mouse/keyboard to interact with the MAC. Not perfect, but it works quite well and it's free.
I've seen a program called Synergy that probably does what you want. I believe LAPD was using it when I heard about it. You roll the mouse pointer off the edge of one screen and it appears on the other monitor. The keyboard control follows the mouse.
I feel your pain. I am currently working on a Linux/Windows setup of four computers, 6 monitors. I use a combination of Stardock's Multiplicity and X-Windows to work from one keyboard/mouse/sound card. Unfortunately, Stardock doesn't support Macs.
Multiplicity is like Synergy--as the mouse leaves the edge of one screen/system, it automatically switches to the next screen/system. And it does support multiple monitors on each system. Sound from outboard systems can be piped to the keyboard/mouse attached system.
Cut and paste between systems works like a champ. I sometimes do things like copy an error message on one system, mouse over to another system and paste the message into Google search. That way, I can keep the entire error situation on the screen and do an exact search on another.
It also supports drag-and-drop of files between systems. I don't use this feature all that much but it's nice to have it available.
For Linux systems access, I bring up X-Windows on a Windows system, then use Multiplicity between Windows monitors.
I highly recommend Multiplicity for anyone who has to work with multiple Windows systems.
Maybe you could configure a remote access to the Mac from another Windows system running Multiplicity? It's relatively inexpensive--something like $40 to support 9 computers.
"There are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies, and the other way is to make it so complicated that there are no obvious deficiencies. The first method is far more difficult." - C.A.R. Hoare