Hi RS6MRA
To be frank, and not to criticise what's already there, whether the wrong side of the door had the handle fitted or not, you have to deal with the problem, except I don't see it as
too much of a problem. Sure you could buy a new door, but that would require not just expense and hassle fitting, when in fact personally I have another way to tackle it, which I believe will result in the door continuing in use:
If it were my own job, I'd get some wooden batons
(2 or 3) that I'd insert into the hollowed-out door, and stick them in position
(using your preferred adhesive), to form something to screw into, behind the outer surface, so that you have some thicker, more substantial wood, into which you can screw. You could effectively insert wood for most of the width; and what screw holes
don't provide anything of much substance for the screws to bite into, will be much strengthened and supported by all the others - and there seem to be quite a few.
Now you have adequate 'backing' for the screws to hold on to, and have more than adequate strength between them to maintain structural integrity.
If you were to adhere the wooden batons either side
first, finishing with the centre piece
(which you can put a small screw into, to hold it while you position and 'pull' it towards you, so there are no gaps, and hence the screws have something to bite into, then it should be good, and solid.
Then when fully dry, you can 'gently' drill out the centre, where the squared metal shaft would go. But remember to
first hold that wood with screws
(the ones used for the handle itself) to hold, as you apply pressure to drill out the centre hole.
This is indeed salvageable, and not that difficult, or costly, to do.
I hope it helps?
* Sorry guys, it's quite a late post this one, and I didn't see all the others, so you have probably now sorted it out!