Concrete questions, if i may ask:
-do you/your GFs leave (small) diamond jewelry at home - eg rings? semi-precious stone things - eg small earrings /necklaces? if one is mugged, do they ask for all wallet (ie credit cards/ID/cash) or one has time to pull out cash to hand over? do they stay there while you remove your jewelry??? these are silly, i truly don't know. i do know not to fight back.
I wear a diamond ring every day as well as another quite noticeable ring. My porteña friends think I'm crazy to wear them, esp the diamond but honestly, they're not common here so I think people assume it is fake 
I don't however wear gold jewelry or an expensive watch as those 2 things do seem to attract attention from muggers. In re mugging - typically they're going to snatch your bag (motochorro or grab it from a table). If they actually mug you on the street with a knife, they have usually targeted you in advance and will say "give me your watch/necklace/whatever" (or at least that's what happened to my friends.) For example, my friend had his Rolex stolen. We were pretty sure the waiter saw it and called a friend b/c 10 minutes after he left the restaurant, he was walking down the street and the thief came up behind him and said "Give me your Rolex." They knew specifically what they were after and there's no way they would have known he had it on randomly since he had on a long-sleeved shirt that covered the watch. Rolex/Cartier watches by the way, pretty much a guarantee you're getting robbed if you wear one.
- say you want to wear pretty quality shoes (not flashy/designer) shoes or clothing - does it attract attention?
I wore my very expensive designer shoes everywhere. I stopped b/c the streets are crap here and walking in 4+ inch heels is a recipe for disaster on a cracked sidewalk. Plus your nice shoes get ruined b/c the heels get marked and mangled from all the cracks in the sidewalks. Heels do get noticed here (designer or otherwise) b/c they're not super common due to the terrible sidewalks. After swearing I would NEVER wear flats, I have become a flat wearing girl. My pretty shoes sit in the closet and cry
same for bag - eg mine is lime green patent leather, medium size, yet it's old and not logo - would that stand out too much? and am i right to prefer carrying it under arm vs over shoulder or in hand?
Again, I use designer bags. No one is going to notice. The trick is you want to have hold of it in case someone tries to snatch it on the street. So if you have a strap, put it across your body. If you are carrying a clutch, keep a good grip on it.
- and again, keys/moneys - if you do need to leave your purse at your table - do people have some strange inner pockets/corsage hiding places?
Don't ever leave your purse at the table.
