I'll take a quick crack at it. It will be in general terms, since I am going by memory from stuff I last looked at at least a decade ago.
1. Please use D2EHPA in your research and reference. "DEHPA" isn't correct.
2. Indium extraction commercially is usually done from single acid solutions (typically sulfuric or hydrochloric systems). It can also be extracted from nitric solutions. The mix of nitric and hydrochloric complicates things. I guess if you are extracting from aqua regia and that is your only choice, realize that ion bleed through (chloride AND nitrate ions) and speciation (indium forms a lot of different complexes depending on pH and counter-ion, and having both chloride and nitrate acids makes for a mess) will make generalizations very difficult.
3. indium extraction improves at "higher pH" generally, when extracted from HCL or H2SO4 or HNO3. For nitrate and chloride acids, high pH is still below 1. A better description might be g/L of free acid, since pH measurement is iffy below pH 1. If I was testing this, I would start at maybe 10 g/L free acid.
4. D2EHPA is a fantastic general purpose extractant, and works very well for indium. By that, I mean fairly good kinetics and decent equilibrium recovery level. Still, a typical extraction circuit would require 3 mixer-settlers for extraction (running loaded organic and loaded feed solution counter-current) and then at least 2 stripping cells (again, running counter current). It might be possible to use a column SX unit since the kinetics are pretty good for indium extraction, but I suspect that a person would have to test that. Also be aware that loading indium onto D2EHPA releases acid into the aqueous solution, and depending on the initial concentration of indium this may require you to neutralize the solution between extraction unit operations e.g. mixer settler #1, re-neutralize pregnant leach solution, mixer settler #2, re-neutralize solution, etc etc.
5. I would strip with HCL at perhaps 50 g/L to start, depending on the acidity level at loading. So, if you loaded at 5-10 g/L acid, you might try stripping at 50 g/L. If you end up having to load at 20 g/L acid, you might need to strip at 100 g/L. Obviously don't go too high in stripping acidity, or you will destroy your organic solution. You could then recover the indium by sponging with zinc, drying the sponge, melting the indium sponge and casting cathodes, electro-refining, remelting anodes with ammonium chloride to purify, and casting final product. You could end up with 99.9% indium if done carefully.
About selectivity - you are right that it will depend on loading (feed) pH/acidity. Generally speaking, at pH below 1, you can selectively extract indium versus most base metals (like zinc, nickel, cobalt and cadmium). I can't say for sure how selective D2EHPA is for indium vs. the PGMs on the basis of pH. What I DO know is that PGMs are experimentally separated using D2EHPA by changing the concentration of D2EHPA in the organic phase. Platinum can be separated from some other PGMs by using LOW D2EHPA concentrations in the organic phase. This will complicate things since you will also have to tune the D2EHPA concentration for indium extraction efficiency.
Also remember that you will have to play with dilution (D2EHPA must be diluted in kerosene) and possibly phase modifiers (such as octanol or decanol) in order to get organic/aqueous phase separation. If you don't get that part right, you will just end up with acidic mayonnaise, which you won't appreciate.
Just thoughts dragged out from my ailing memory.
Best Regards, Geraldo