Re: Mac and ipad syncing
I have multiple Apple IDs for when I need to get apps only available in certain countries. I stick with my UK one but once a month or so I use iTunes, click "Account Info" and then simply log in to one of my other accounts. I click "check for updates", update any apps that need updating, then move on to the next one.
A sync with the phone or ipad and it's all done; you don't need to switch Apple IDs on the phone or ipad itself (unless that's the device you have with you when you want to grab an app from some other country).