Having big problems syncing iMac, iPad, iPhone

Trying to sync my mum's 3 devices without any success most of yesterday!

Upshot is I seem to be able to sync either iMac and iPhone or iMac and iPad but not all three at the same time.

Have so far tried iCloud and Dropbox - different problems with each. I don't think wifi syncing is available yet is it?
Have deleted apps on iphone and ipad and reinstalled.
Removed save file from Dropbox. Switched off syncing, then replaced file, turned syncing back on, in various different orders and combinations.

The history of symptoms might shed some light:

• First of all we were using v3 on iMac (bought direct from Agile), and latest (v4.2x?) on iOS.

• Don't know how we were syncing (probably Dropbox) but Mac and iPhone were OK, and then iPad became completely blank/inaccessible at some point around time new iOS was installed (was working back in June).

• iPad 1P was not recognising Dropbox login, and then when it did recognise it, it couldn't find the 1P file. Tried many times. Lots of error messages. It kept trying to sync via iCloud (which we weren't using).

• BTW, the iPad 1P had a different master password from the iPhone/iMac 1P master password - is this allowable?

• So at that stage iMac and iPhone working OK, iPad openable but blank.

• Went to iMac / iTunes and there were a bunch of apps to download/update, but when I tried, some couldn't be updated "because they were associated with a different account", the new 1P iOS app was one of these.

• This looked like an old problem which was rearing its ugly head, which I think started many years ago when two almost identical email addresses were used by accident as Apple IDs (one with a "." ; one without)

• So I deleted all the offending iOS apps from iMac/iTunes and the 2 iOS devices, but of course you can't delete them from the iTunes App Store.

• Then tried reinstalling on the iOS devices - successfully.

• Tried Dropbox again, same problem as before: "wrong login" for Dropbox, then when Dropbox finally opened "iPad can't find 1P file".

• Then downloaded, but so far not purchased, new iMac v4 1P, and tried syncing via iCloud.

• first with iMac, then iPad : lo and behold SUCCESS! Syncing fine between those two!

• Then tried iPhone, but now it's the iPhone which can't find the iCloud sync file and throws up error messages! So now iPhone has an empty 1P but iPad is fine!

• Then I thought I'd try here for any suggestions…