Your small regular will pretty much cover your small sebenza needs. The Insingo blade shape isn't so drastically different that there is a lot that one can do that the other can't. If you were to get another small sebenza, you might check out a damascus blade, inlay, or graphic to mix up your collection.
But I say go for the Mnandi. For a while I thought it was too small, then one day it clicked for me and now I love it. I went through the same progression with the small sebenza actually. The Mnandi might be my favorite knife design. It is simple, beautiful, and elegant, and can handle most EDC tasks. The more CRKs I collect, the more I realize that all I need is one large Sebenza, one small Sebenza, and one Mnandi. Those three can cover pretty much any situation I can come up with. And the Mnandi is especially perfect for the situation you described (office, light EDC).
If you find yourself in the market, just know that the older Mnandis have a different nail nick, pivot, and pocket clip. I've seen preferences both ways, but I greatly prefer the older nail nick for both form and function. I am unable to open the newer Mnandis with just my thumb (have to pinch it open). The older Mnandis open easier then any Sebenza I've owned. Some prefer the new pocket clip because it is sleeker and looks better in the pocket, but just know that If you want to take the clip off, the new style requires you get an extra screw from CRK. The old style can be removed without any extra hardware.
