Hi there,
I've fitted rear belts in my Fiat 500 to allow fitting of children's seats. I have a forward facing infant seat and a high back booster seat.
I can post some pictures once I get home to the car. I used a set of front Fiat 126 inertia belts.
I presume you already have rear belt mounts? I didn't have any so I welded in a set of these:
https://www.ebay.co.uk/itm/Land-Rover-Series-Defender-Seat-Belt-Mounting-Stress-Plate-7-16-Captive-Nut-x2/282747113478?hash=item41d50a9006:g:JPgAAOSwDEtaGEkx:rk:4:pf:0This would be a good installation for the 126:
http://car-from-uk.com/sale.php?id=90867&country=uk (Image 4)
Fitting the belts in the door bin, moving up to a shoulder anchor on the rear parcel shelf. You have a bit more rear door bin space then I do in the 500 so I had to do something a bit different.
Those belts should be good. there will be a date of manufacture on the orange label on the setbelts. (Top tip!). If you're happy visiting a scrap yard, seicento/cinquecento seatbelts will also fit.
I'm not sure about isofix.
There are a number of aftermarket kits to adapt a normal seat to an isofix seat:
https://www.ebay.co.uk/i/323538116478?chn=psThey use a strap attached to the top of the seat to hold the isofix bracket in place at the bottom. In a crash the weight of child seat is transferred to the rear upright seat, which in turn transfers to the seat mounts. These were never designed to carry an load in a crash.
The bracket could be welded in place to the rear bulkhead, but it will need a spreader plate to handle the load and it may clash with the seat fitting.
I've found a pair of rear child seats fit ok, but it works best with the rear bench seat removed to give some visibility through the rear window.