baby seat help - fitting lower rear clasps to avoid 'buckle crunch - urgent (ish) :)

i'm in the process of having to buy my mum (in her 70's) a baby seat for our little Darcey as soon my mrs will be going back to work and to help with child care costs my mum is going to have her a couple of days a week.

As my mum has an occasional bad back we've been quite taken with the Maxi Cosi Axiss Buy Maxi-Cosi Axiss Car Seat, Total Black online at John Lewis

Its good because she doesn't have to lean in to the car with the baby in hand as the seat can be turned towards the door and then rotated back when she's firmly strapped in. This will really help her as the last thing I want is my mum to put her back out especially if shes on child care duty and stuck in a car park somewhere.

The problem it would seem is that my mums little C3 has slightly too much length on the rear seat belt clasps and as such when the Maxi Axiss is fitted and tensioned correctly the current clasp leans towards & touches the seat creating a pressure point which is known as 'Buckle crunch'. This was also the case on the next 3 seats we tried but in honesty they're not as usable for my mum anyway.

Apparently cars with recessed rear clasps are better for avoiding this as there's less lean in the clasp and the belt does its work.

Buckle crunch info here - Halfords Advice Centre | Car Seat Fitting Guide

So, I need a cheap yet viable solution if we want to get this seat.

Ive looked on ebay and noticed the C2 rear seat belt clasps look much shorter in length and on a fixed bar.

My idea is that I swap out the current C3 clasp and put in the lower anchored C2 clasp thus reduced the lean and effectively creating a slightly recessed seat belt point.

Pictures below of a C3 clasp and measurements to bolt hole (confirmed on my mums car):



And this is a C2 rear clasp at which Ive guessed the measurements and hope to fit:



Now, working on the theory that both clasps will fit the actual Citroen seat belts and that the bolt hole is the same size, can anybody see why this wouldn't work or more importantly would it likely create a safety hazard?

I'm simply trying to reduce the length in the most logical way, so swapping pre-manufactured safety parts (if they fit) is in my mind the safest way instead of having a part shortened etc.

Would it effect the safety of the belt do you think or in theory a good idea? Is lowering the anchor point likely to cause the belt to not work correctly?

i see no reason why it would but obviously 2nd opinions from people much more mechanically minded would be nice.

 

did that on my Audi (well I fitted them), looked in to getting my mums car done but the mounting points aren't there for retro fit bars on her model.