Argentina appears to follow the common rule that you must apply for a visa at the consulate that covers the area/country where you live.
Your nationality determines whether you need a visa at all or not, but once you have determined that you do need a visa, it is your residence that governs where to apply.
You don't write where you live, but if you live in Nepal it looks like the embassy in New Delhi is where you need to go to interview to get an Argentine visa.
The practical reason for this rule is that one part of considering your application is to determine whether you're likely to leave Argentina after your trip and return to Nepal. To this end you will need to demonstrate convincing ties to Nepal that would motivate you to go back -- and the consular officers in New Delhi will be the ones with training and experience in evaluating ties to Nepal and with the kinds of documentation that is reasonably available to Nepal residents. Those in Bangkok won't have this experience, so they won't be able to give your application as meaningful an evaluation.
It seems to be correct that as a Nepalese citizen you can get visa-on-arrival in Bolivia. However, of course, that would enable you to visit Bolivia, not to go to Argentina. Whether you think this convenience makes it worthwhile for you to change your vacation plans so you visit Bolivia instead of Argentina, is something only you can decide.
A Schengen visa will probably not be relevant in either case -- except that eyebrows will be raised if your entry/exit stamps show that you overstayed it.