1)
I tried to compare them once in the Apple Store, but it was a crowded day and they were on different sides of the table, so I did not do an extensive pixel-peep comparison. I think I saw a slight difference, but it for sure did not strike me like lightning by just having a quick glance at the screen. If you go to an Apple store, they have iPhoto and Aperture installed with sample pictures so you can see for yourself.

2)
One other difference (IIRC) is that the Retina model does not have built-in ethernet, you would have to buy a Thunderbolt-Ethernet adapter if Ethernet is important to you.

3)
Yes, the Retina applies only to the laptop screen. External monitor is what it is, regardless of the model you choose.

I personally went for the non-retina model because I edit my pictures most of the time on the external monitor, and I don't miss the extra SSD speed. Maybe I would if I did a lot of Photoshop or video editing, but coming from a 2007 Macbook (white), the current MacbookPro is lightning fast even without SSD