The short answer is "Yes, of course." There are different aspects of CONOPS to be represented, and this can be done in a single, or multiple diagrams. In general, CONCOPS requires that you identify the players - what are the structural elements or roles that collaborate in the CONOPS; this can be done on class/block diagrams or on sequence or activity diagrams with swim lanes. Then you want to talk about how the operations unfold, that is, what is the operational activities performed by these players during the operation. This can be shown on sequence or activity diagrams quite easily. In general, sequence diagrams are best for showing single threads of execution or with at most a small number of variants. Activity diagrams are richer and can show a greater set of operational variations quite easily. If I were to use a single diagram, I would tend to use an activity diagram with swim lanes identifying the contributions of the important roles to the flow of the operational concept. However, if I wanted to show the structural elements, their relations, and their interfaces (and possible information flows), I would augment this with a structure diagram (UML) or IBD (SysML).

The required degree of completeness of your CONOPS should dictate whether or not you include "rainy day" cases. I certainly do for the primary failure points in a CONOPS but it depends on the need your model is trying to meet. If you do include rainy day scenarios, this can be done on a single activity diagram with decision points, a single sequence diagram with interaction operators, or multiple sequence diagrams, each showing an operational variant.

BTW, your note suggests you have a couple of misapprehensions. First, a use case is not a behavioral diagram (no matter what the SysML spec says); it depicts functionality but not how that functionality is achieved with behavior. Secondly, sequence diagrams need not show internal functionality at all; you can decide the level of detail and scope to include in the sequence.

I hope this helps.

- b