Open with happened in Yesemite DP and now El Capitan.
finder has been glitchy up to DP 6 now... DP5 had file associations all jacked up.
as far as copy paste... i make sure to use the appropriate keys for the os i'm in and havnt had a problem. cmd vs ctrl...
Preview crashes here and there when i fly through a pdf...
the issues appear for me given enough time, no guest booted.
Its a preview/beta... in both cases all is expected. but i'd point towards apple for your issues. And the releases on EC have been a bit odd, pretty close together, so i'd say they're working some issues out.