I'm the only poster who has ever used "serio-comic," but I've used it three times. Well, now four.
t.r.a.g.i.c.o.m.i.c.a.l.l.y was used once but then that post was quoted. I don't know how we're handling that (might be one of the forms Saxo has mentioned)
Some retractions made above. Although t.w.i.z.z.l.e is a valid word (both singular and plural) the validity of those particular incidences I question upon revisiting. Perhaps it is one of a series of thoughts on the nature of words under consideration that we can parse.
The post for T.w.i.z.z.l.e would seem to be a nickname for the musical artist T. Swift. That for t.w.i.z.z.l.e.s appears to be a typo for the licorice brand. Both require some interpretation that makes me wonder what exactly the singleton is that was originally posted...
As for p.a.l.i.n.k.a, turns out it was the file name of the image, which I suspect this falls outside any scope of discussion here.
Now, for a further question, I present n.a.c.r.e, which has only appeared twice, once in English and once in French. Am I misguided in sensing there are actually two words here despite their sharing a common meaning and spelling? If words from other languages are to be under consideration, there are probably many more singletons to be found.
To your second point, first: I feel more strongly about including the category of re-quoted posts, as it indicates a sole original use whatever the relevance others found in it. The downside is that a search yielding multiple results (due to quotes) would run contrary to the spirit of Googlewhacking that served as an inspiration.
I am more conflicted over the "external quote" where there is no clear indication the original poster had any hand in the particular word aside from introducing the text as a whole. On the other hand, I am partial to the idea that external quotes often expand the t.e.x.t.u.a.l.i.t.y of the forum.
As for the first point, I am sure it will not come as a surprise to you that at least anecdotally my searches show a number of terms are the domain of a single member or only a few members.
Right, there is confusion about the best and most enjoyable way to play this new game.
Gori's idea to find words that appear
one time was a great idea!
I would expand the rule list as follows:
1) Use the site search engine to find a word that has only appeared in a single OT post.
2) Spell your word out with spaces, periods,or post a screen shot, so as not to spoil the one-post beauty of the search results by making your post in this thread itself a second instance of the word's use.
3) It has to be a real word in the English language.
4) Words added after April 18th 2022 don't count. It is as if they don't exist.
... new rules?
5) A word shall only be counted once if the poster quotes themselves (even if they quote themselves multiple times).
If another poster quotes them, it counts as twice and is not a singleton.
If a poster uses the word in 4 separate posts, then it counts as 4 uses.
6) A word quoted from an outside source like wikipedia or a news article shall be counted.
7) A word that only appears in the name of a link such as p.a.l.i.n.k.a won't count.
8) A word that is a real word in the English language, but was misspelled or misused by the poster, shall be counted.

9) A word that is a real word in the English language but was spoken in a different language shall be counted.
Reasoning:
5) A poster who quotes themself is usually expanding on a train of thought, so it is really expanding the 1st post instead of making a 2nd post.
Being quoted by another poster introduces a new thought and use of the word, so the word should count twice.
A poster using the same word in multiple different posts is also introducing new thoughts and uses of the word. It didn't really make a singular appearance in the OT forums which we prize.
6) We mostly parrot other peoples' genius ideas around here to make ourselves seem more genius, so an offsite quote that generates a singleton should be honored.

7) Winning a singleton this way seems dirty and not fun.

8) The mighty search engine does not pass judgement on the posters. Neither should we.
9) The English language assimilates all other languages over time. Just like x.e.n.o.b.i.o.l.o.g.i.s.t. is not a real job that exists (yet), so too are foreign words a temporary phenomenon.
If it shows up one time, it should count!
If a mistake or foreign language causes more than one word usage, tough luck!
Perhaps the players can reach a consensus for maximum fun?
Ha! Stupid Norwegian guy found one;
c-l-a-q-u-e-u-r-s
It's mine though. Is that cheating?
Ya, it is past the cutoff date.
Perhaps try the singular?