28" and 26" motors are normally the same. I think the labels are more for the retailers and wheel-builders so they don't get them mixed up. À Most motors come in a range of different windings. Sometimes just two - one high speed for small wheels and one lower speed for big wheels. The Bafang BMP has a code stamped on it for the winding speed, and I've personally seen 9 different codes, so you can pick one to suit your exact requirements if you can find a supplier with the one you want. The MAC also has a wide range of windings.
Yes, approx 20% of the length, which normally means a blob on one of the upright legs. Keep the soldering iron in contact with the shunt the whole time. Keep it there until the solder becomes wet and fuses with the shunt. Make sure that the iron is fully up to temperature before starting.
Procedure: When the iron is up to temperature add a bit of solder to the tip to aid contact. Put the tip on the leg of the shunt and keep it there for maybe 5 secs or so to heat up the shunt. Keeping it there, add some solder between the tip and the shunt until there's enough to cover it. You can move it around with the tip, but try not to lose contact with it. When you can see that it's all melted in nicely, remove the iron and admire your work.