As mentioned in the comments, this is an intriguing question (or sets of questions). Let's address the different aspects brought in by you, one by one.
Is this a nonce word?
Spanish differs from English in quite a few things, one of the main differences is that it has an organism which acts as a "language authority". You will see several mentions of the Spanish Royal Academy (Real Academia Española) and its dictionary (El diccionario de la Real Academia de Española) when there is a serious discussion regarding words, terms and usage of the Spanish language.
Why is this important? It is important because this particular organism sets what is the closest to a "Language Standard" and that affects the way "nonce words" would be handled. All nonce words are neologisms, and neologisms are handled by the Spanish Royal Academy to be "officially" recognized in the Spanish language, such was the case of "cantinflear". An important criteria for a new word to be recognized is how widely used the word has become, in order for a word to be widely used, its meaning should be self-evident so that it can propagate among the Spanish speakers.
Having said that, nonce words in Spanish are not that common. A nonce word might be used informally, mostly verbally, if it becomes popular enough it might eventually find its way to become a neologism and finally become recognized by the Spanish Royal Academy, which will lead to its inclusion in the Spanish dictionary. Only then, will that word find itself used in an article that can be considered "formal" such as a news article.
So, considering the fact that this word was used in a news article which reports on the official examination results performed by a police surgeon, the word could not be a nonce. It is either a formal medical term or a typographic error.
With this in mind let's take a look at your next question:
If the word were taken up in more widespread use, how might Spanish writers/speakers use the adjective "hemicular", outside a medical context?
A typical Spanish writer/speaker would use the dictionary to find what the adjective "hemicular" means, then they would make a decision if the word is clear and specific enough to use it instead of a well-known synonym. Another driver would be to avoid "vocabulary poverty" (pobreza de vocabulario) when writing an article, which would encourage them not to repeat the same words over and over.
Now, let's take a look at your third point:
I wonder if that question may possibly be determined by its etymological roots. Is this word related to hémico (equivalent to the English hemic: "of or relating to the blood or the circulatory system") ... or is it rather composed of the two Latin roots "hemi-" (half) + "-culum" (a diminutive or instrumental suffix)?
Considering the etymological roots of "hemicular", we have two options:
- Hemicular: Relative to hemiculo
- Hemicular: Relative to hémico
If we take a look at hemiculo, it would be a word composed only by a combination prefix and a suffix: hemi- (half) and -culum (small) which does not make sense. The English equivalent of this word would be akin to "hemicle", and the phrase reported would be "half-small line".
If we take a look at hémico, then "ular" is redundant. If we consider hémico as "Relative to the blood", then "hemicular" would be "Relative to the Relative to the blood", the closest thing making sense would become "Relative to the relation of the blood". In this particular case, the phrase reported would be "the line relative to the blood", which would be closer to "blood line" or "the line following the veins". If this was the intent, then the phrase in Spanish should have been "línea hémica" instead of "línea hemicular".
Given that you mentioned you went to google translate to read the comments regarding your question, you can see that the native speakers tried to find the meaning of the word by either querying the dictionary or inferring it by context, which would be the first step in considering the usage of the word (as stated above when looking into point 2). Also, you must have noticed that all of them failed to find a result. If you query the Spanish Royal Academy's dictionary you will not find "hemicular":
La palabra hemicular no está en el Diccionario.
La palabra hemiculo no está registrada en el Diccionario. La entrada que se muestra a continuación podría estar relacionada:
And, even more shockingly, not even "hémico":
La palabra hémico no está registrada en el Diccionario. Las entradas que se muestran a continuación podrían estar relacionadas:
Since the Spanish Royal Academy's dictionary is the dictionary of the "Spanish language authority", then that means the word does not exist. Since hemicular should not be a nonce word, then it must be a typographical error.
Now, the question is "Isn't hémico a Spanish word?" This is another interesting question, even though there have been cases of people using it in blogs, the correct word recognized by the Spanish Royal Academy would be:
Del gr. αἱματικός haimatikós.
- adj. Med. Perteneciente o relativo a la sangre.
While hemínico is also used in on-line medical publications, it is not in the Spanish dictionary, either. Apparently, this has also confused a few people who have translated hemic system.
While translating "hemic system" to Spanish using Google translate yields "sistema hémico", the proper Spanish translation is "sistema hemático". There are few hits when you search for "sistema hémico" but a few are Portuguese which could explain the confusion between some Spanish speakers that use google as a replacement for the Spanish dictionary (being suggested to use words from other languages close to Spanish, such as Portuguese).
Lastly, let's address the following:
En forma pràctica se debe tomar el D1: si es positivo nuestro eje elèctrico se encuentra en el hemiculo izquierdo y si es negativo en el hemicirculo derecho.
This paragraph might not give us all we need to derive its meaning from context, so let's take more from the same source:
En forma pràctica se debe tomar el D1: si es positivo nuestro eje elèctrico se encuentra en el hemiculo izquierdo y si es negativo en el hemicirculo derecho (-).
Luego tomamos aVF, si en el trazado ECG es positivo, nuestro eje ocuparà el hemicurlo inferior y si aVF es negativo la incripciòn ECG, ocuparà el hemicirculo superior.
When we look at both paragraphs we can tell that they are instructions to determine where the electric axis is located, summarizing: if D1 is positive, it will be left hemi-circle, if it is negative it will be right hemi-circle, if aVF is positive, then it will be bottom hemi-circle, if it is negative it will be in the top hemi-circle.
"Hemiculo izquierdo" is a typographic error, since both should refer to the "hemi-circle", there is no official "hemiculo" word that Spanish people would use only to refer to the left side, just like there is no official "hemicurlo" word used only to refer to the bottom.
As additional evidence, a search for the word "hemiculo" throws a result for an IT related paper with the following paragraph:
El siguiente paso en el algoritmo del hemicubo consiste en proyectar sobre las paredes del hemiculo el
resto la escena tomando un punto de vista centrado en el punto central del hemicubo y mirando cada vez
a al centro de una de las cinco paredes del hemicubo
The paragraph describes an algorithm to project a scene on the walls of a "hemi-cube" (hemicubo), making a reference to "the walls of the hemiculo" when it clearly means "hemicubo", the hint that perhaps there are some typographic errors in the work are right above the paragraph, in its heading:
Proyección e la Escena sobre las paredes
This clearly should be "Proyección de la Escena sobre las paredes". The example above just shows that there are a few technical papers on-line where some typographic errors occur. Also, when being more strict, the use of the french grave accent is incorrect in Spanish "ocuparà" should be "ocupará", showing that the article in question is not adhering entirely to proper written Spanish.
In conclusion, the usage of "hemicular" appears to either be a typographical error or an incorrect term used by the police surgeon who could have used "hemática", instead of "hemicular" (not "hémica"), if the intent was to use "hemic line".
In this particular context, I believe Abraham's comment regarding "línea hemicular" being a typographical error of "línea hemiclavicular" is what makes the most sense; however, there are a few examples of medical publications using "línea hemática".