Hello,
this is very interesting. In fact, as far as I can tell now, QuickOPC with Excel option seems like a good fit for this.
About the only issue I can foresee is the fact that for it to function well, the connection to the OPC UA server will have to work at the time somebody wants the value. Of course normally the connection should be operational, but they need to think about the scenario when it isn't, and how would that affect their workflow. The Excel sheets can store the last retrieved value (if they assure that the sheet gets saved) - either with the formula cell (?) or if they somehow arrange that the last known value gets copied elsewhere. I suggest they make a "proof of concept" and simply try it out.
Unless they specifically need to drop in new cells to their spreadsheets on the Excel workstations, Connectivity Explorer is not required to be installed on them. In fact, only a fairly minimal installation (there is a choice for it in the Setup) is needed - just the QuickOPC assemblies and the RTD server COM component. All that the Connectivity Explorer does is that it puts together the Excel formula, which then gets copied or drag&dropped into Excel. Once it is there, Connectivity Explorer is not needed any more. In fact, the formulas can be put together by hand as well, eliminating the need for Connectivity Explorer.
Technically, both license keys (one for QuickOPC and one for the Excel option) always need to be installed on the Excel workstations (the "Excel Options" does not enabled anything by itseld, it just extends the base key).
As far as the license conditions go, this is bit of a gray area, as the definition of "Developer" is not as clear with Excel as it is with, say, C# development. I would label anybody who creates or modifies Excel sheets with RTD formulas pointing to QuickOPC as a Developer. So, if, on the Excel workstation, they just use the Excel sheets prepared by somebody else (with the Developer license), or modify them but without modifying the RTD QuickOPC cells, they would be OK with treating the Excel workstations as "Runtime" (and will therefore not need to pay for them extra). If they need to modify the RTD formulas on the workstations (and especially if they plan to use Connection Explorer for that), that each such workstation would need to be covered by a Developer license (=Single Dev. + Runtime free, or Additional Developer, or by applying a Site License).
Best regards