Titular: Relating to the title.
Eponymous: Giving a name to.
Both are not only appropriate, but often used, as such. Strictly if the collection had been given a name first, and then afterwords the story written for it, then eponymous would be wrong, but that's not the case.
I'd go for eponymous, just because I think it's the phrasing that would come to mind first, and I don't see any reason why I would decide to alter it afterwards. I wouldn't see anything wrong in titular either.
I do understand how attributive and predicative adjectives work, yet I can't find an easier way for learners to differentiate between both types - especially if such differentiation doesn't exist in their mother tongues.
I'd like to suggest that maybe a reference grammar might be able to help you here. For instance, the 2002 reference grammar by Huddleston and Pullum et al., The Cambridge Grammar of the English Language (CGEL), discusses this very issue: "Chapter 6 Adjectives and adverbs", part "4 Restricted function for adjectives", on pages 553-562.
CGEL, in "4.1 Attributive-only adjectives",
- Adjectives that do not normally occur except as (heads of) attributive modifiers include:
which includes an example list of 30 adjectives (e.g. drunken, future, mere, putative, umpteenth). In this part, they also discuss "Meaning differences between attributive and non-attributive uses".
They also discuss "Potential differences between attributive-only and ordinary attributive adjectives"; where they discuss the four properties that ascriptive attributive adjectives characteristically have, but most attributive-only adjectives lack one or more of them. The properties are: entailment, subset, modifiability, pro-form.
Then they go on and discuss "Some types of attributive-only adjectives". In this part, they say:
- Attributive-only adjectives are too numerous and semantically heterogeneous to permit a simple and exhaustive classification. Instead we will here illustrate and comment on some of the most important semantic types.
Some of those types are: degree and quantifying attributives, temporal and locational attributives, associative attributives, process-oriented attributives, modal attributives, particularizing attributives, expressive attributives, hypallage: transferred attributives.
CGEL, in "4.2 Never-attributive adjectives", discusses "Adjectives which can occur predicatively or postpositively, but not attributively". These include "Adjectives formed with the 'a' prefix", and provide an example list of 24 adjectives (e.g. afloat, aglitter, alike, averse).
Also, includes "Adjectives with complements". And also includes "A small set of other adjectives".
And then includes "Postpositive-only adjectives", which are described as:
- A handful of adjectives are restricted to postpositive function:
and they provide an example list of 6 adjectives (e.g. flowers galore, the President elect).
Okayee, my fingers are tired. Hopefully I haven't made too many typos. It seems to me that CGEL has a lot of good info in those pages that might help you out (pages 553-562).
Best Answer
Title is the noun and titular is the adjective, as you know and (subjectivity warning!) I would encourage using adjectives as adjectives and nouns as nouns*.
There is, however, precedent for using the noun-noun construction.
Since the 1960s use of titular hero has declined while title hero has increased.
This perhaps follows the use of title hero which is far more common than titular character and has been for a long time.
*I would appreciate comment, edit or additional solution which addresses the part of speech of title in title hero. I expect it is not a noun marauding as an adjective, but I am not well enough versed in grammar.