21.5 inch retina iMac used as external monitor
Yes, in the sense you can share the screen over a internet connection. There will be a noticeable small delay as information is exchanged over the internet connection. What you would normally see on the iMac is the same as seen on the Mac Book Pro. The displays would mirror each other. If you had an actually external monitor connected to the Mac Book Pro and you are not mirroring the displays, then what you would see on the single iMac display is what is shown on both the internal and external Mac Book Pro displays. So in this case, the iMac would mirror both the internal and external Mac Book Pro displays. 

No, in the sense of an actual external monitor. There is no input jack on the iMac like you would find on an actual external monitor.

Other than this, I do think it is possible for you to buy another computer.