Ordering of Particles:「にや」OR「やに」?

Question

Does the particle や come after particles like に or で (as does も in「にも」or「でも」), or does it come before them? I can't find any examples of either, be it through Google; in the Tatoeba database; or otherwise.

Answer 1

や in the sense that you are using it, is a linking particle between 2 or more nouns, and cannot be left "hanging" after a noun or noun phrase, so you would not have to worry about how to connect it to other particles.

If you want to imply "etc." or "and others", "and so on" after a single noun or noun phrase, then など (or などなど) would be more grammatical. Depending on the circumstances, とか might also be acceptable.

YES: 一等親や友達には
YES: 一等親や友達などには
YES: 一等親などなど(の関係人)には
NO: 一等親やには

Answer 2

You would expect very little possibility to see the coordinative や and the case particle に next to each other.

や factors out the shortest different sections, so you will find [particle]+や in a XやY composition that X and Y ends in different particles.

{東京や仙台]から来た人 (diff. nouns w/ same particle)

在籍期間が[2012年まで‌や2018年から]の人 (diff. nouns w/ diff. particles)

That means the theoretical order would be にや, but in reality, you will be not likely to encounter such a situation, because に is one of central case marker in Japanese, and や stricter about syntactic leanness than its kinds* (such as か, とか and やら), which means, you need another particle, that is different than に but compared to に, to make something like にや appear. That would result in a very terrible type of zeugma, like conjoining subject and object together.

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* cf.

じぶんがいじめられたら先生に‌とかともだちにそうだんする。
× 先生に‌やともだちにそうだんする。