Michael,
Your market share projections are pretty close to what I would say. A nationwide threshold of 35% and no significant increase in share on any route would allow UA to combine with just about any US airline except AA or NW and AA to combine with just about anyone except UA, DL, or CO. There are significant possibilities for combinations in the airline industry without triggering the usual market concentration thresholds used in other industries.
We as a country do tolerate market concentrations as high as 80% when doing so is necessary to support the hub and spoke system – which is an acceptable way to serve small markets throughout the country. In fact, many arguments have been made that cities like CLT and CVG would never have the level of service they have if an airline were not allowed to have such a high percentage of the market.
The corollary is that the most concentrated hubs like CLT, CVG, and DFW (which also happen to have some of the highest average fares) all have plenty of capacity to handle competitors if they choose to serve those markets. There really are very few remaining examples of markets in the US that are highly concentrated, have no low fare competition, AND have no capacity to handle additional service should competitors choose to add it.
For those of you except Michael who could care less about this sidebar, the point is clearly that the industry is capable of consolidating along several lines. As such, I believe the US government is willing to allow market forces to work during this phase of industry restructuring rather than block consolidation or intervene in the industry when competitors are close to failing.