It would be easier to answer the question if you could say why RoboHelp was ruled out as an option. The only person who would need a license for this or any other HTML Help compiler is the person who will be compiling the help file. If it's a matter of cost, you can get the free HTML Help Workshop from Microsoft here:
http://msdn.microsoft.com/en-us/library/ms669985.aspx
Being free, however, it requires a lot more work and tends to be a lot less intuitive than the commercial help applications.
As far as integrating with Excel and being intuitive to the end user, an HTML Help file is an HTML Help file regardless of the software used to create it. The vast majority of Excel integration can be handled with a single API call that is described in some detail here:
http://www.help-info.de/en/Help_Info_HTMLHelp/hh_api.htm
How well this works depends a lot on how well structured the help file is. There are also some issues related to difficulties closing the help file cleanly if the user shuts down Excel with the help file open that have not been clearly documented anywhere I'm aware of.
Whether or not the help file is intuitive to the end user is a design problem that really just requires experience and good intuition. I've written quite a few help files and I still don't pretend to be more than just adequate at this aspect of the job.