I'm not particularly taken with the photo, but I also know how hard it was to overcome a flow of "wow" effects in Nepal and Tibet. There's just too much, and managing to find a subject and frame it, too, is really well done, IMO.
Let me first congratulate you on an excellent critique. You've highlighted some important problems which we all tussle with. Namely, the magnificent scene before us, which promts us to take the shot, frequently looks 'lack-lustre' in the photoshop rendition. It often seems to fail
to capture what we actually experienced.
I'll go through your point one by one.
1) The banners, which become an annoyance in the BW conversion.
Banners? What banners? I'm not aware of any banners. What are you talking about?
2) The sky, which is dull in both the colour and BW versions.
Fair enought. That can be easily fixed.
3) There is too much detail in the mountains for a small web version, at the very least.
This is another issue entirely. We all suffer from this problem; ie. how to represent a magnigicent scene in a low resolution jpeg format. In some instances it's probably just not possible.
4) The complexity is too great; the foreground simplicity works well, but the background lessens the effect.
As mentioned in item 3, this is an unavoidable effect of small jpegs.
5) I don't think there's enough contrast for the BW conversion.
Valid point. I'm not a B&W expert. I almost never produce B&W images from color data.
So what would I try to do?
I've cropped to a square format, and I've applied a BW gradient map and a curves adjustment. (The effect is perhaps a bit exaggerated, the monitor I'm sitting at currently may be a bit too bright, but I don't have anything to verify that with at the moment.)
Whatever improvements in contrast and tonality you may have achieved, the loss of the panoramic effect is more signiant.
For those who are interested in detail, and I am for sure, below is a 100% crop of a section of that image, in color. Fairly large file.
I quite like this, except that I'm not satisfied with the structure in the mountains, and I'd also wish that there was some interesting shadow structure to bring forward. Perhaps there is with the raw file. I'd lose the pole, but by now it's almost invisible.
I might have bungled here, but I can't remember the circumstances of each of the 12,000 shots I took on this trip.
The fact that there is acreage of redundant sky seems to imply to me that at the time of the shot, I had made a decision that the lower part of the scene was mere clutter.
However, as a matter of interest, here is the shadow detail, in color.