Even with the last 6.7 version of ACR when opening to CS5 from LR4 you only have limited compatibility with PV2012. It seems to honour the new controls in the basic panel, but if you choose to open as a Smart object, you find that you only have the PV2010 controls available. As a consequence I never use open as Smart object. Also as I have noted before at times I also seem to suffer from stair stepping artifacts around non vertical/horizontal very high contrast edges. Some times it is so bad that I have to resort to having LR export the image to disk as a 16 bit PSD, which is then perfectly fine. It is obviously due to the fact that PSCS is obviously using ACR to render the image that is opened, not using LR.
As to being able to open 6D files in PSCS 5, I do not know how your school/college/university has done this but Adobe do not and never have supported new cameras with ACR updates for "old" versions. Once a new version is out that is it no more updates for things like new cameras. ACR 7.x will only run with PSCS 6.