1) Yes the number of OGMs should be 'limitless' and I shouldn't see any problem with about 100, however I never stress-tested the software to find out.
2) So long as you have enough bandwidth and the computer hardware, it should support about 100 callers fairly well, however some users have reported difficulties after just 3 or 4 simultaneous calls. I don't think this was for everyone, but certainly some did encounter difficulties and this may have been fixed in the latest version.
3) There are various services out there that do stress testing on your VoIP applications for a fee. Just give them the number to call, how many calls you want sent and it will call in as many times as you want with as many simultaneous calls as you want. Can be a bit expensive though, so you typically only do that in your final stages when your application is ready to be deployed.
4) I can't think of any published examples. Most companies do not release such information however, so you would really only know if someone just happened to casually mention the size of their IVR app on these forums somewhere.
5) I have no idea, the best I can suggest is to try stress testing yourself.
6) Unfortunately, you assume all liability and there is no guarantee (implied or otherwise) that the software will meet your requirements for a system under such demand. You can purchase one of the support options and try to get better support from NCH if you run into a problem, however other than that you are on your own.
This why I suggest you do some testing on your own first to make sure IVM will be able to meet your demands before you spend time (and money) on developing and deploying an IVR application just to find out it doesn't work out for you.
Note: I do not work for NCH nor am I an official representative for the company. Any information I provide is accurate to the best of my ability, however could still be inaccurate and I assume no responsibility (nor does NCH) for any misinformation in this post that may lead to any problems later on. You assume all risk and responsibility associated with using the software.