For what it is worth, here is what Cornwall's look like sitting about 6" off the back wall. The wall they are on is about 14 feet wide (they are centered on the wall) and that is a 55" screen on the one on the left (some day I will get around to getting it on the wall mount). I often listen at low levels and they perform well.
I am sure that Cornwalls or Fortes can sound fine 4-6" off the wall. That is not to say that they wouldn't sound better with more space behind them. The Cornwall's would likely perform better fully against the wall since they are front ported. As already mentioned, the Fortes have a radiator on the back and you will want a some nominal amount of clearance so you don't hamstring them.
If you haven't already, I would encourage you to make it a point to listen to a Forte or Cornwall before taking the dive. I bet they are going to sound quite a bit different than your Infinitys.
You asked about what to look for in used speakers and I would say it depends a lot on which version you are looking at. There is a significant price difference between the original Forte and Forte IV. However, it is also my understanding that there are also noticeable improvements between most generations. If you are looking at Forte I/II, it is not uncommon to find them with modifications, especially Crite's tweeters, but I wouldn't say that it is typical. As a DIY person, the common modifications to the I/II's are well documented, easy to perform, and usually reported to generate notable improvements.
If you are looking into III or IV, I would guess the majority of them will be stock.