Whichevereth does not appear to be listed in dictionaries.
With only a few Google hits, across a selection of informal texts and snippets, it is perhaps used to indicate that the speaker does not care about the ordinal position of the object in a long row thereof:
And indicating with a nod a bespectacled and nonthreatening Mancunian solicitor with whom he was apparently vying for the latest Louis the whichevereth bureau, [...]
[Alex Ferguson's email to Cristiano Ronaldo]
In this sense, it seems to be a cliché, largely synonymous with umpteenth.
However, whichevereth is perhaps not immediately understandable in this sole sense (i.e., as a pronoun pertaining to order). Indeed some speakers use it as an archaic form of whichever:
And ye shall face the snorkel whichevereth way thy boltholes lineth up or there will be great gnashing of teeth.
My questions:
whether whichevereth is largely understandable in the sense pertaining to order
whether it can be used in a question (such as, a question about position in an order).