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It's not free, but not expensive either: I have been using Synergy for years for exactly the same purpose. It now costs $19 for the basic version, which is probably what you want. The Pro version for $49 will get you also clipboard sharing (and a lot of other things which are still in development), but for the basic keyboard/mouse sharing the cheapest version should be fine. I believe they have a money-back guarantee too.
If you really want to squeeze out the last penny, the source code for the core component is available on GitHub. You will have to compile the C++ code yourself and check out the docs and code to find out how to configure it. I would probably go with the 1.8.8 version in that case.
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.