On the umpteenth time it finally restarted but it's only temporary so I'll start doing this stuff tonight.
There is no data on it which originated on it - or very little. Proper backups are impossible as the Mac won't talk to it except via Android File Transfer.
What I will do is a series of screen shots to remind me what apps I find most useful though presumably whatever corruption has happened may be down to them anyway.
If it is factory reset, what will the Play Store do once it finds a "naked" phone? Presumably the fact that it knows some apps are installed or even paid for is a factor of the phone and not the Store?
Yes I know you can do it with Android File Transfer. First remove Kies altogether.