The rear facing seats for older children are differently shaped to baby seats so children are not squashed in them. In fact the ones which accommodate children up to approximately six tend to have the option to fit them with a bit of a space away from the back seat, forming a space there for a footrest.
My 3yo forward faces now, but on a long journey he tends to pull his feet up into the seat and cross his legs, so I would take that to mean that dangling feet are actually the part that is uncomfortable.
Legal crash testing takes place at 30mph. The idea that it's 15mph is a misconception - two cars crashing into each other at 30mph causes a total force of 60mph that is divided by two - 30mph - it's not the 30mph that is divided by two. (In fact it's more complicated because the weight of the vehicles ties in as well - the heavier vehicle will take on less of the resulting force and the smaller more, proportionally).
Also - just because a car seat is tested to 30mph as the legal standard, does not mean that it will definitely fail when crashing at higher speed. Some of the very cheap very flimsy seats produced with nothing other than cost in mind have been known to do this, but it's not a given. It's actually quite a difficult thing to design a product to withstand a 30mph crash with an 18kg child sat in it. Once you've got it to do that, it is likely that it will offer some if not substantial protection at higher speeds as well. Especially the models that are designed with safety in mind because the brand has a reputation they want to protect.
It is true that younger children have more vulnerable spinal cords and they are more likely to suffer such injuries in a forward facing seat, but forward facing seats still provide protection against the two biggest risks in a car - being ejected from the car and hitting their head on things inside the car. (The second dependent on the seat being fitted tightly and the harness being fitted tightly - rear facing seats also do this better). The spinal cord injury risk is a small percentage of outcomes. It's up to anyone to weigh up the risks and benefits for themselves. If you absolutely want to minimise/rule out the risk of that specific outcome happening, then it's beneficial to rear face until age 4. And some people are. If you are not that absolutist about risk, and some people are not, then forward facing might not bother you. IME these two groups of people seem to struggle to understand the other, which makes very little sense - we can understand that people have different attitudes to risk in other areas of life, but car seats seem incredibly divisive, possibly because of the emotionally manipulative way the ERF message is pushed in some circles.
I also agree that it's not about whether the OP's request is reasonable or not (and I think it is) it's about the MIL deliberately going against it and being secretive about it. I do notice though that a lot of people are coming up with reasons why the MIL might be finding rear facing hard - would it be worth a discussion about it? Maybe there will be a way to alleviate the problem so that both parties are happy.