Dude I take back everything I said about the 705 half soles. Those look killer! I may very well follow your lead, in fact! That's like a perfect compromise for a lowere heel. ( also, you're making me reconsider medallion vs standard cap toes
I personally like the natural edges and think the color contrast would be awesome on a navy boot. Yes they're pretty easy to clean, but mind you they will patina and darken up a lot with age and wear. But for me that's half the fun:)
To your question on ankle support, no idea. You could also get 6" bounty hunters for a little extra, but my guess is you'll be fine. You said something about flat feet. You do know Whites are all built with a high arch last, right? Maybe it's a good thing or maybe it's a bad thing... Fwiw, I have pretty flat feet and used to wear orthotics for planter fasciitis- I find whites to be incredibly comfortable (and for me it only took minimal break in).