IMac – Use iMessage on Mac without signing into iCloud in System Preferences

1. No. iCloud requires Windows Vista SP2 or Windows 7.

2. Data will have to be synced manually, yes.

3. Yes, manually syncing iTunes will not interfere with iCloud. All devices connected to iCloud will sync automatically and update their contents in the cloud after they have been manually synced.

I had a similar issue and eventually got sick of dealing with the issues you have noticed and gave in and used the icloud account. Calendar, app store, reminders, notes, etc. It all works better with the icloud account. I never use the email address associated with it (and I've had one since the @me.com days).

Yes it is annoying but Apple's built-in apps work much better with Apple's accounts. You can spend hours of your time troubleshooting or moving over to dedicated 3rd party apps or just "grin and bear it."

I know that doesn't solve your problem, per se. And there may be folks who have found acceptable workarounds (if not here try macintouch.com) who can offer a better solution but I found that not fighting city hall ended up working for me rather well after all... YMMV...