Teleport not secure is only an issue of you are using it on a public network or at work where you do not trust your fellow employees. At home, you only need to worry about other family members.
Teleport has not been updated in years. That it works is at least nice, and gives you an idea of what you can do.
You might try Synergy, even though it is paid for app. It is not too expensive.
The next level would be a hardware KVM (Keyboard, Video, Mouse) hardware box, and external USB keyboard and USB mouse (or a wireless mouse that uses a USB dongle that can be plugged into the KVM). Then depending on the KVM device, you either use a keyboard hotkey to switch between the 2 computer systems, or you have to press a button on the KVM to switch focus.
A physical KVM is going to be more expensive, than Synergy, and most likely less functional.
As for screen sharing. It is not that much bandwidth, and it only updates the screen when something changes. Also nothing says you can't use a dedicated Ethernet cable between the iMac and the Macbook Air (USB to Ethernet adapter would be needed for the Macbook Air).
At least give Screen Sharing a try. It can be done today, and you never know, you might find it "Good Enough". I say this while typing on my iMac keyboard, using my iMac's Magic Trackpad, typing into a Screen Sharing session hosted on my iMac connected to my Macbook Pro.
Back in the Snow Leopard days (10.6) I used Teleport, and it was useful, but that was before I got my iMac and I was using a much smaller screen on my PowerMac (PowerPC based) cheese grater Mac. I played with Teleport after getting the iMac, but found it simpler to use use a Screen Sharing session.
