You use the tag yet you avoid the term intertextuality in your writing. There is little in literature and in the practise of criticism or analysis that is "immutable" and amounts to being "laws". The various concepts of intertextuality that were put forward since J. Kristeva coined the term address your concerns.
Manfred Pfister proposes, for example, the following criteria of
intensity:
- Referentiality (quoting; offering critical commentary etc)
- Communicativity (intensity of being conscious of intertextual relationships, regarding both author and audience)
- Author's reflexivity
- Structurality (structural parallels, matches, identity of pre-text and text)
- Selectivity (level of abstraction; selection of single, isolated aspects vs larger similarities)
- Dialogicity (semantic and ideological contrasts)
Other structural or post-structural flavours of theories of intertextuality (e.g. By G. Genette) may use different terms.
When thinking about intertextual relationships between texts we usually deal with a "continuum" or a "spectrum", sometimes even in extreme cases. Taking the list of criteria above, plagiarism, for example, would score very highly on the structural criterion, yet extremely low on the communicative one, since plagiarists usually do not communicate the fact that their text was copied verbatim from somewhere else.
You are certainly left at liberty to discuss the degree of intertextuality between texts and make a case for a strong or a weak relationship, for one that operates on significant levels or one that operates on mere "echoes" or incidental parallels as long as you can back up your views with evidence from the text(s).
Also note, that regarding the intertextual "afterlife" of literary characters, there is also a sister concept: Interfigurality