The Mini Copperhead is a fairly small knife. 3 1/4", and both blades use the same backspring. Very low profile, which makes it nice for carrying, but perhaps not as nice for using for extended periods. Your had will probably cramp.
The Sway Back Jack is also fairly small, but is more substantial than the Mini Copperhead. 2 backsprings, obviously, which makes it thicker, plus a larger handle. Most find it very comfortable to use because of the handle shape, despite its short length. It's also fairly hefty for its small size, due to its construction.
The Seahorse is a much more substantial knife than the other two. 3/4 of an inch might not sound like much, but when it comes to these kinds of knives, it does. It might be larger than you'd care to carry in your pocket. The Seahorse will probably weigh about double the Mini Copperhead.
There are other options, unless you're insisting on buying a Case. Northwoods makes a 4" dogleg whittler with a wharncliffe main blade,
and is in D2 steel which will hold its edge far longer than Case's steel. If you're OK with carbon steel (not stainless), check out Great Eastern Cutlery, specifically the patern #61 Congress (4 blade) and Half Congress (2 blade), the pattern #62 Half Congress (2 blade) and Courthouse Whittler (3 blade), and the #89 Executive Whittler (3 blade). I believe some #61 Half Congresses were made in 440C stainless. Many other companies make/made congresses and half congresses as well. Personally I'd stay away from the Case ones though, like the 64052 (4 blade) and 62052 (2 blade), as the blades on these are rather thin and flexible. Maybe the 6488 has beefier blades, I don't know (it's a much larger knife).
As for the purpose of the coping blade, in carving, it's used to scour lines and do fine detail. Coping blades are typically thinly ground so they're good for removing material a little bit at a time. They also shred cardboard boxes nicely.
Here's a good link for the various types of blades you see in traditional pocket knives.
http://www.agrussell.com/Knife-Encylopedia/a/134/
Note that what's usually described as a coping blade is referred to as a "cut off pen" blade in that link.
EDIT: Oops, I just noticed that the Northwoods Dogleg, unlike their other whittler, is not D2 steel, but 420HC. Probably 56-57Rc hardness. Still probably better than Case's Tru-Sharp (which is supposedly also 420HC, but a bit softer). Northwoods does have a small congress in D2 in some beautiful stag. 4 blades: 2 large sheepsfoot main blades, a pen blade, and coping blade. A bit on the pricier side, but stag always is.