Yes, it is long but useful
I don't think you will get anywhere with a yum update all.
Split it into three: The OS, Plesk and Dell OMSA.
Start with the OS (so disable the plesk repo, the art repo and the dell repos).
If that goes without a hitch, enable the art repo and try again.
Then try the Plesk side again. I can see there are going to be problem with that though, and no different to when you tried updating everything at once. I'm not an expert with this -- you might want to get some paid for help with it -- talk to Breun, 4psa, Parallels etc -- but I get the feeling that there's some stuff in there from an old plesk install. Do you use all those modules (e.g. the gameservers and multiple language packs and things)? If not maybe try to remove them.
Leave the dell OMSA until last.