I used to play violin and piano seriously, but I was just a junior high student, so there really wasn't that much other stuff getting in my way. It sounds to me like you're much more passionate about the guitar, and would never consider dropping that instrument, whereas piano is something you'd like to explore and add to a list of options. While I felt like violin was much more natural for me, I always kept in mind that I had more experience on piano, and that was going to be my main instrument. I also let my teacher know this, so that when piano competition time came up and my violin work started to slack, he knew why.
As for the time commitment that two instruments takes, you really have to enjoy what you're doing. While music should challenge you, you have to see it as entertainment and a passion rather than work. Play around with your schedule, and see how things work out. But keep in mind your priorities. For me, piano always took precedence over violin.
Taking on more instruments allows for a better understanding of music. I find myself applying my violin knowledge to my piano playing quite a bit. If I had the chance to pick up the violin again, I would. I've always regretted ever leaving it behind because it opened up lots of opportunities for me like playing in large ensembles and busking. So now that you have the option of picking up piano again, you should try it out. If it doesn't work out, at least you'll know that you didn't miss out on an opportunity.