[wonger_]

I haven't tried a treadmill. I recommend running 50-100m barefoot on solid ground like others have said and seeing how it compares / how your form changes.

I usually run 1-2 miles when I go out, not too much. Just enough to feel all the secondary leg muscles get fired up. Of course if you're new to barefoot running, people recommend starting with short distances.

On shin splints: one trigger was my poor running form. I would lunge my feet forward focusing on getting the longest stride, landing on the heel because my foot was in front of me. It didn't hurt my heel because traditional shoes are so padded there, but the physics/angles/forces of that form are bad. Instead, barefoot runners talk about "falling forward", which is exaggerated imo but gives you the cue that your feet should land under or behind your torso, not in front. This reduces the impact on the feet and legs. Barefoot running teaches this foot placement very quickly. Not sure if that's something you already do or not, but hope it helps.

EDIT - just saw your other comment. Go for a little barefoot run, maybe a few different times, and I think you'll quickly find out where your feet want to go. The "torso first, not feet first" mindset really helped me. (I'm flat footed too, and insoles didn't help me personally; not to yuck your yum, though. Maybe they'll be more comfortable for you)