Q1: Does that mean that I cannot use the libxl.net.dll in the Appeon client?
A1: Appeon Mobile: NO
Appeon Web: Should work via Appeon`s .Net assembly support
(as long at there is no visual aspect to this DLL).
Q2: Does that mean that I cannot use the libxl.net.dll in the Appeon client?
A2: Have the Web Service return a Blob. -OR-
- Do what I do in my imaging applications & save the datastream to a Blob column
- Then use a Select Blob directly from your Appeon code
Note: Using this approach you can support Appeon Mobile as well. 
Q3: Can I have the client save the spreadsheet as comma delimited and simply import into a datawindow for processing?
A3: YES