Please help me better understand / correct my misunderstandings of, the details of societies which used shells as currency.
It seems to me that, even if the shell-traders are only interested in a certain distinctive kind of shell, it would be a relatively short time before the glut would make a single shell nearly valueless. But I haven't heard of shell trade being abandoned because of inflation.
I am under the impression that shell trade often made made its way fairly far inland. Would societies necessarily be considered more shell-poor the farther inland they are? If so, why would they value shells on par with the societies closer to the coast?
It seems to me that the societies which harvested shells would necessarily be powerful or influential because of this, but I haven't seen anything which directly relates shell harvesting to regional power. Why is this?
Your points of interest seem to be derived from some underlying assumptions; that there is an unlimited number of shells available to be collected, that the collection of shells and their transformation into currency is comparatively labor and cost free compared to other economic activities and that wealth arises directly from the production of a currency rather than from the production of goods and services that can be purchased with a fungible currency.
While waiting for an answer that directly addresses each of your questions, which involve economics as much as history, you may be interested in this earlier, archived, thread which covers how shell money worked and broadly covers some of your points.
https://www.reddit.com/r/AskHistorians/comments/6hjgjc/problems_with_shell_money_eg_cowries/