When there is no copula and nothing seems to be omitted, as in your second example, I interpret the sentence as simply a fragment, used to "set the scene", and would translate it as such. It is very reminiscent of how fragments are used in descriptive passages in English:
Pale druggists in remote towns of the Epworth League and flannel nightgown belts, endlessly wrapping up bottles of Peruna. . . . Women hidden away in the damp kitchens of unpainted houses along the railroad tracks, frying tough beefsteaks. . . . Lime and cement dealers being initiated into the Knights of Pythias, the Red Men or the Woodmen of the World. . . . Watchmen at lonely railroad crossings in Iowa, hoping that they'll be able to get off to hear the United Brethren evangelist preach. . . . Ticket-sellers in the subway, breathing sweat in its gaseous form. . . . Farmers plowing sterile fields behind sad meditative horses, both suffering from the bites of insects. . . . Grocery-clerks trying to make assignations with soapy servant girls. . . . Women confined for the ninth or tenth time, wondering helplessly what it is all about. . . . Methodist preachers retired after forty years of service in the trenches of God, upon pensions of $600 a year.
-- H. L. Mencken, taken from here
Compare the above with this short Japanese passage that is clearly going for a similar, if not identical, effect:
何者かが何者かに切り殺されている気配。嘆き。叫び。 … 血塗られた背と折れた剣、砕けた柱……。
Hints of someone being slashed and slain by someone. Wails. Cries. ... A back smeared with blood and a broken sword, a crumbled pillar....
And your example translated in this way:
The convenience store that has grown while limberly changing shape, 40 years since its inception.
The form and function seem quite similar. There's practically no limit to how much the noun can be modified, be it adjectivally, adverbially, relatively, or phrasally. And what we get in effect is a snapshot, a picture, an isolated concept. The focus is not on what the noun is, does, did, etc, but on the very noun itself.
When the copula is present, as in your first example, I don't see how the sentence is any different in form from 「私です。」, as in response to 「誰ですか？」, only with additional modifiers:
It was a Hana who couldn't say anything to Kayo.
And before I get bashed for abusing English, let me just say that modifying proper nouns--and pronouns!--with the help of definite/indefinite articles is a well-established practice:
Mr Dursley hummed as he picked out his most boring tie for work and Mrs Dursley gossiped away happily as she wrestled a screaming Dudley into his highchair. -- J.K. Rowling
The me I was yesterday is no more. Let the me of tomorrow come!
ASIDE: Brandon's suggestion is definitely more natural and would work in practice. However, I feel that it's more a translation of the following:
And there was Hana, unable to say anything to Kayo.
With there-insertion, was is no longer the linking-verb be, but the existential be.
EDIT: How does the Hana sentence fit as a narrative of the scenario? Why the relative-noun-copula structure?
Again, here's the sentence in question, with what I believe to be the omitted topic:
subject + が doesn't seem likely here.
I don't believe this sentence is making a statement about Hana. If it were, it would instead be 「はなはかよに何も言えなかったんだ。」, your suggestion. Rather, it is a statement on the general state of affairs, quite fitting for an ending narration.
The distinction may not be obvious. To illustrate, I'll appeal to English's It, as the general-state-of-affairs idea is inherent in certain usages of It.
Consider the "weather it":
(1) It is raining.
(2) It is cold.
What might It be referring to?
In (1), one might argue that It = Rain,
Rain is raining., where to rain means to fall. (Interestingly, this is how (1) is expressed in Japanese: 「雨が降っている。」)
However, the idea that the "weather it" refers to something concrete is problematic, especially in (2), where it is not obvious what that something is. The weather is cold? The air? The surrounding area? And yet everyone seems to understand the sentence perfectly fine! Thus, some linguists argue that It here simply refers to the general state of affairs.
Also, note how (2) is expressed in Japanese: 「寒いです。」, an
X + copula construction. If we accept that Japanese's
X + copula is similar to English's
It + copula in that both can be statements on the general state of affairs, then it would not be too much a stretch to say that the difference between 「(それは)かよに何も言えないはなでした。」 and 「はなはかよに何も言えなかったんだ。」 is akin to the difference between "It was a cold day." and "The day was cold."
Hence, "It was a Hana who couldn't say anything to Kayo." (The general state of affairs: a Hana who couldn't say anything to Kayo.)
Evidence from literature:
It was the best of times, it was the worst of times. -- Charles Dickens
それは すべての時世の中で最もよい時世 でもあれば、すべての時世の中で最も悪い時世 でもあった。