Steve,
The only sound advice I can give you is, if the powers to be, which I am assuming is a board of directors, have doubts, than get a second opinion. They seem to understand the importance of working with professionals and are doing just that. There are just too many factors to be considered that have not been revealed to make any final conclusions. We can ASSUME many things, such as, proper soil physical testing has been done by a credited laboratory and their recommendations are to create four inches of capping. Jeff is correct about the water release curve and the two projects that I was involved with that capped, the labs were recommending 16 inches for one course and twelve inches for the other. We don´t know just how high the ph is in the soil or the water source. The carbonate content is also important. What is the primary soil and corresponding drainage characteristics. What has been going on the last 20 plus years? For example, if the salts and carbonates were high was a sulfur burned used to treat the water? Was a vertidrain used for several times a year to help flush out the salts during the little rain that the area receives? Was gypsum applied twice a year? As Mike pointed out, lots of bad soils can be chemically altered or one can inject an acid-based fertilizer in these situations directly into the irrigation system and by pass a lot of soil chemistry. If the soil and salinity is so bad that they need to sand cap, have there been any trials with paspalm for a fairways grass or even for greens. You could change fairway grass a lot cheaper than capping fourteen fairways. Capillary concrete in an area that receives six inches of rain a year seems like an over kill but if the membership has had a chance to evaluate it on the other course and there is a notable difference that there willing to pay for, so be it! Has the sand ever been changed in the last 20 plus years? Do you have a lot flash bunkers, that erode and contaminate the sand with fines from the native soil?
Your two professionals most likely have taking into consideration these factors and are making recommendations based on those findings. If the board is skeptical of those findings by all means get a second opinion from another qualified consultant! A couple of grand more for a, four to five million dollar investment, seems logical to me.