If you want to know more about Fe uptake through roots, google "strategy 1 & 2 iron uptake"
As for foliar Fe uptake. Some plants let the chelate (partially) enter the leaf, strip the metal (Fe2 or Fe3) and dispose the chelate back into the water column. Others take up the entire chelate, strip the metal and process the chelate inside.
Which plant uses what method and what the efficiency is, is unclear to me.
I've got a bunch of different iron chelates, and in general Fe-EDTA was enough to have good growth. But the addition or using only other chelates, always did something extra to the plants. Like with everything, some plants react better than others. The last couple of years i've completely banned Fe-EDTA, resulting in more efficient iron use.