With the caliper on the back face, braking force is pushing the caliper into the fork. Put it on the front, and it's pulling away from the fork, putting all the load on the 2 little bolts and the glue that holds the nuts in.
The calipers on my bike (and also on cars with disk brakes) are held on by bolts that are loaded in shear. The axis of the bolts are parallel to the wheel axis, so it doesn't matter whether the caliper is being pushed towards the fork or away. The bolts are equally strong in either direction.
How do your brakes mount to your fork? Flat mount just bolt in like this:
As you can see, the bolts holding the caliper onto the fork are not taking any braking load when travelling forwards. If you were to apply them while travelling backwards then you would be putting the entire braking load through the threads of the bolts.
The ones on my trike are used in shear (with an adapter onto the brake caliper itself), but that comes with it's own alignment problems and is going to be really tricky to achieve with a carbon fork. I think this is the best picture I have of that:
2016-08-24_09-04-17 by duncancmartin, on Flickr
Although mine are suspension forks, and the caliper mounting flange is cast into the fork, the bolt direction is this:-
http://www.bobjacksoncycles.co.uk/product/reynolds-853-disc-brake-front-fork/I agree that alignment is a fiddle. Mine has a selection of washers that pack out around 1 mm between the caliper and the flange.
Mine are quite old, and I think that many have moved on to flat mount.
However, on flat mount, the bolts should still be strong enough to take the braking force. Bolts are supposed to be tightened to the point where there is always tension in the bolt, to reduce the likelihood of the bolts coming loose and fatigue failures.
It's not a big deal. The worst grade of M5 steel bolts is good for about 1/2 tonne in tension (
https://www.engineeringtoolbox.com/metric-bolts-minimum-ultimate-tensile-proof-loads-d_2026.html) so a couple of those will hold the caliper on fine.
Actually, I think that the flat mount calipers can still put the bolts in tension when braking is applied. The mounting face of the caliper is a couple of cm to the left of the disk, so there is a torque trying to twist the caliper around a vertical axis. That is being resisted by the quite narrow mounting face of the caliper, which will apply more pressure nearer the disk and less pressure further away from the disk. The tension in the bolts will stop it turning in that direction.
I don't think it's a problem. Larger bosses could easily be used if it was an issue. But I also don't believe that it would be a problem to put the caliper in front of the disk, where the twisting forces would be exactly the same, so the strain on the bosses would be very similar to what they are now. I think that the calipers are behind the forks to keep them out of the way.