Sundar-ji -
Thank you for raising an important question.
I'd like to take a stab at answering parts of the question; I'm afraid, however, that I'm not certain what exactly you are referring to, so parts of my answer are likely to be off-target with respect to your intended issue.
1. By "this region," I'm assuming you mean primarily the Solu Khumbu area, as that was the subject of your very useful MRD article [which I've linked to the resources page of BridgesNepal.com].
"Sustainable" is a buzzword that can be interpreted in many ways, none of which I'm particularly fond of. The basic point is generally that many human-impacted systems are being run in a way that (without important change) will lead to unsatisfactory conditions in the future. The concept is fine if you are referring to discrete factors such as soil condition, fuel supply, or pollution. What I object to is the rather paternalistic assumptions as to what conditions need to be sustained. Livelihoods, cultural traditions, demographics, and many other factors can and will change.
In general, I don't think tourism should be considered a sustainable industry in most locales. The factors that make it useful for the rapid propagation of wanted changes (economic opportunity, access to health services, education, and so on) are frequently self-canceling. Assets such as exotic landscape and culture, for instance, depend on isolation, lack of ethnic assimilation, disparity in host/guest standard of living, and most important, ambient poverty. At an intermediate stage of development, these assets can be eroded without loss of market, as tourists are intrigued by the paradoxical presence of pool halls, Internet cafes, chocolate croissants, and tea houses run by families who own several cars in Kathmandu and have children studying in the best schools around the world -- all in a tiny enclave surrounded by medieval living conditions.
Once a certain level of prosperity and sophistication is reached, however, the host community will probably tire of the importunities of nosy trekkers. For their part, tourists are likely to feel that the subservience level (never very high among Sherpas, anyway) is inadequate to sustain the fantasy of other-worldliness. Broader economic opportunities mean that rooms are no longer given away for a few cents (literally) just to encourage consumption of food (also extremely cheap), and newer accommodations are not much different (in facilities and pricing) from what is available in the "real world."
So, in general, tourism should be a bridge to development, not necessarily something that is to be sustained indefinitely.
On the other hand, there are places with such unique assets that tourism simply cannot die. Khumbu is probably one of those. The Sherpas will never recover their privacy -- unless the country falls prey to the sort of instability that has descended on Kashmir.
One of the least sustainable commodities, in fact, is a consensus as to what needs to be sustained. The Sherpa dialect of Tibetan is now essentially a "second language" as far as many young Sherpas are concerned, a useful "private language" for talking behind the backs of outsiders but no longer the most expressive means of communication. Television, radio, newspapers, polyglot commerce, and immigration are sure to continue the process of marginalizing Sherpa language -- and other aspects of the culture as well. Many of the more established families are now anxious to conserve what remains of the culture, and there has been talk of setting up a Sherpa-medium boarding school at Lukla. (Does anybody know what's happening with that?) Unfortunately, Lukla is probably too far down the slippery slope of assimilation. With cheap fares to Kathmandu, not to mention the huge tourist impact in Lukla itself, it will be very hard to maintain Sherpa traditions.
On the other hand, it may be argued that one of the central Sherpa traditions is an openness to change. The Tibetans who picked themselves up and emigrated from eastern Tibet some 500 years ago were self-selecting those traits that made them formidable traders and entrepreneurs. As Sherpas, they continued the tradition of innovation. Potatoes, for instance, are a comparatively recent introduction, now much more important than barley -- which would probably disappear from Sherpa areas if it were not wanted for moonshine production.
In terms of the impact of tourism on the ecosystem, there is no simple answer. As Alton Byers has shown, the Khumbu area was very early on heavily impacted (and stabilized) for the sake of animal husbandry. The rapid increase in demand for fuel when tourist arrivals skyrocketed in the seventies obviously had an impact on forests, but a greater impact was probably caused by the gazetting of the region as a park and the removal of the shingi nawa system of forest protection. Fear of deforestation itself contributed to a rush in house construction and hoarding of wood. It seems that the situation is now stabilized, primarily due to the Sherpa's own perception that forests need to be conserved. This perception has not extended to other less "charismatic" vegetation systems -- Aton has been calling for more protection, particularly, of the vegetation above the natural tree line. That kind of positive influence, of course, is typical of the back-and-forth impact of tourism. It simply doesn't make sense to look at trends at one point in the dialectic and say, "See! It's unsustainable!!!" Current practices and trends may well be unsustainable in some respects, but with an educated host community I think we can be pretty sure that the situation will not become super-critical. And that education is a direct result of tourism -- as exemplified by Ed Hillary, and the thousands of mountaineers and trekkers who have followed his lead, both in their fascination with the landscape and in their respect and friendship for the people.
[More later]