First of all, you have provided very little information about the terrain. One option is that you should find a spot where you have a clear line of sight to all the other access points and put a 5 ghz omni in there. Then, at the local base stations, you install one 5ghz directional panel and a 2.4ghz omni for the clients. Another option is to install four 2.4ghz sectors in one spot (assuming you need to cover a 360 degree area around it). But without some additional info about obstacles it's impossible to say what would work better.

As long as this technology is patentable, corporations will not allow it.

You are never going to see 100mbit/s over wifi using indoor equipment. You might get 50-60mbit/s real throughput if you're lucky. Personally, I would stay away from all-in-one boxes and get a wired gigabit router running openwrt to handle all the routing and nat. Then, you can get any generic 802.11n router and configure it to run in bridge mode. You can also keep the wrt54g for 2.4ghz, running in bridge mode as well.

The lack of a requirement to operate under a license will only make a mess of the available spectrum, exactly as it is with both 2.4 and 5.8 GHz wifi. In places where there are many competing WISPs, nothing actually works, because all of them are interfering with each others’ access points. The licenses should be essentially free, but at the same time the number of operators in a given area should be limited. The thing about rural areas is that there are not many potential customers there, so installing a 10.000$ professional radio relay for 10 customers is simply not viable. The hardware needs to be cheap, the license needs to be free and the company that would get the license should not have to worry about interference from their competition, because otherwise it’s not worth the WISP’s trouble and they drop those problematic locations quite quickly.

The largest problem by far is that he did not state what kind of budget he was given. I really wanted to give him some advice, but with such vague requirements you could write a fairly large book on the subject and still not provide an adequate answer.