Your message says "Can we give the vehicle and carnet to a hired local driver to take to Calcutta? Or can the person named as owner (which is different) take it through?" I assume in the second question you meant to say *must* the person named as owner take it through, NOT *can* the person named as owner take it through.
I have been in similar situations. I know how tough it is. I will try to be as thorough and clear as possible. Forgive the candour, I am trying to help, and I hope you will take the following in that spirit.
Regarding the passport stamp - yes it is true that you cannot exit the country without your bike. Legally that is. In practice, you probably can. I had occasion to leave India without my bike and there was no customs check of the passport at the airport - passport control is one thing, customs another. They don't talk usually, and generally speaking, customs are less interested in you as you exit a country, as opposed to entering. The control will be looking at your visa expiry date, validity etc. It is not their job to deal with vehicle export-import.
This is my experience in India, that's all - you could be unlucky and come across someone who understands the rules. Unlikely, but possible.
Crossing a border with a carnet in another's name seems more tricky to me than others have said. I see Maarten's point that you have proof of the bike leaving. Physical, tangible. That's good. The problem - aside from breaking the explicit contract to stay with the vehicle - is that it looks as if the owner may have sold the bike. There is tax to pay on a sale. That was my experience in Argentina - we were in a similar spot to Quark, and, as I suggested to the female official that I take ownership of the bike to ease the problems, she nearly had a heart attack. Really, she changed colour. So it may be a big no no. Nepal/India could be different of course, I am just alerting you to the possibilities based on my experience.
It seems to me that you have three sets if negotiations: out of Nepal, into India, and out of India. Each could need slightly different tactics. Nepal's borders with India are very poorly run. I'd say that you could swing most things there. India is another matter. On some borders the controls are pretty serious. (I don't know what the western ones are like.) And this goes doubly for Kolkata. There, you will have the full uniformed, handlebar moustache, stiff back and shiny shoes lot. The carnet will be checked over by several people. They are very picky in my experience, stupidly so. Am I correct in thinking that you are expecting third party to do all this for you?
Your shipper may not be too happy either - make sure he is primed well in advance.
Are you really going to give the bike to a local you don't know, who will ride/carry it to Kolkata, negotiate the borders and paperwork and then sort out the shippping at the end? That would be, for me, a nightmare last resort scenario. I would count on never seeing the bike again. I would be wanting to be holding some serious security against him on that deal - a family infant at least. Maybe two.
More negative, I'm afraid: when you say "shipping" do you mean air or sea? I have heard first-hand some horrendous stories about Calcutta port from bikers who have shipped by boat from there to the far east. Count on four weeks of running around. Are you really expecting someone else to do this for you? It's not like you just hand over the machine when you arrive and sign the bottom of a piece of paper. Not at all. Surely there is something I am missing here.
You have shopped around for prices from Nepal I assume? For a few hundred dollars extra that is by far your best option for immediate departure. Maybe even a thousand? Yeah, I know. At the end of the day, you may not have a choice. Best to mentally prepare yourself for that eventuality.
Talk to the RAC.
Go to the central customs housevin Katmandu. Persist a bit with different offices. Someone may take pity on you and make it all easy. There may be a solution.
When I needed to leave India without my bike I left it in a bonded warehouse in Delhi customs house. It was in a secure underground car park with dozens of other vehicles. I paid a small fee for storage. Is this a possibility for your friend - to return later and ride the bike out? I would suggest that this is the best option of all, from what I understand of the sitution.
If you do go ahead with this plan, get the letter of permission stamped and verified by a legal professional. In most counties this would be a notary. Doubtless Nepal has a similar functionary. It wont cost much, and it gives you a bit of leverage - customs official don't know the rules any better than you do. They are driven by: a) making their own lives easy, and b) not wanting to get caught out. If you have some big red stamps by a legal firm all over your letter, it gives the appearence that the the matter has been pre-approved, if you see what I mean, and by mighty legal forces in the capital at that. This is a major plus for you. I would get a letter from your embassy too, even if it says next to nothing, if adds weight. So too the central customs office in Nepal. In both these latter cases write it out for them first: "maybe we need a letter a bit like this? What do you think?".
In other words, the sugestion you have put here, as I understand it, does not appear to be a good idea to me. I would suggest either 1, pay up the big shipping fee out of Nepal or 2, store the bike and return later, as your best options for investigation.
I hope this helps.
Around the world 2000-2004, on a 1993 Honda Transalp