It is possible to attach an iOS device and force a backup without doing a sync.
Can't recall the key combo (think its hold down Cmd-Opt) as you attach the device and that should cancel any sync. Then select the device in the left column and right click to do a backup or transfer purchases etc.
HTH
I don't know of a keystroke combo, but all I do is change the settings in iTunes prefs to disable auto syncing, open iTunes, plug device in, then right click on device and select back up.
As far as copying content from the iPad to the Mac, you can't do it in iTunes directly.
Firstly, sign into your iTunes account on the iMac. You should then be able to automatically download any apps and music (and books) you've bought from the iTunes/App store using that Apple ID.
For anything else that you've put on your iPad that didn't come from the iTunes store, you need a third party app. iRip2 is really good for this (
iRip 2 – iPod and iPhone music transfer software for Mac or PC – The Little App Factory) It does cost (the free version is limited) but it works very well.
If you still have the PC and don't want to spend on an app to facilitate copying your content from your iPad to your iMac, then go to the PC, and copy all music and video onto a flash drive/ext hdd, plug it into the Mac (Macs will read NTFS formatted drives, but not write to them) and copy it all into a folder on the desktop.
Then, go to iTunes, open prefs and make sure "Copy to iTunes Media Folder" and "Keep my iTunes Media Folder Organised" are both ticked, then drag that folder on your desktop into iTunes. Let it compile it all, then go to FILE (in iTunes), hold down the ALT (also known as option) key and click "DISPLAY EXACT DUPLICATES. Delete the duplicates (don't delete them all, just one, or more if applicable) of each - if you delete them all you do actually delete them all - you don't want that.
Once you're happy you don't have duplicates, and you have everything you had before, then connect the iPad and sync it - it will delete all apps music and video on the iPad but it will replace it with the same music apps and video thats now in your iTunes, so it will all go back on again.