Due to special circumstances, our Ward is going to be calling an additional counselor for Relief Society.
Adaptability and flexibility are offered by the Handbook, so local circumstances may definitely warrant that. From a broader perspective, I have found that often these types of stresses on leaders and organizations, can be lessened when a re-evaluation occurs of what exactly are the "things that matter most", and then deciding what good things may end up not being done, so you can do what's truly needed. Admittedly, if you're trying to have the Relief Society Presidency completely follow everything listed in GHB 9.3.2.2, many RS Presidencies will be overwhelmed even with capable and experienced Presidency members. And perhaps even when delegating and having functioning ministering secretaries, service committee coordinator and members, activity committee coordinator and members, and assistant secretaries, the RS Presidency may still be overloaded.
We're often at the other end of the spectrum, happy if for a time we actually have two counselors called in the RS Presidency (but still with many many needs). So we constantly evaluate what the RS Presidency is asked to do, and encourage them to be satisfied with what they are able to provide, even if much is left without being able to get done.
Again, not to say the local Bishop and Stake President are wrong if that's their decision, and they very well may have already accounted for these considerations, in their decision-making for your unique situation.