Ir a comprar

[Nfqufktc]

I would appreciate it if you could look at the analysis of the following sentence:

Una chica fue a comprar sombra para los ojos que le diera un aspecto vampiresco.

So far I have seen the periphrasis “ir a haber” to have been used in the imperfect only to refer to the future-in-the-past time.

Allow me to put forward the following explanation of the pretertite of the “ir a haber”:

According to the PRET. Reminder which I have made for my reference:

PRETERIT IS NARRATIVE
------------------------------------------------------------------------
PRETERIT PROVIDES AMPLIFICATION / DETAIL IN TERMS OF ACTION / a CHANGE OF the IMPRESSION/ CONDITION

------------------------------------------------------------------------

ACTION / IMPRESSION (completed / specific / compressed / sharply defined)

Here the form “ir a haber” is used to express INTENTION. It is her intention to buy this product that is COMPLETED (or as you indicated that “that was her intention from the very beginning”, should I infer that the STARTING POINT IS ASSERTED?).
It doesn’t matter whether she bought it or not, what matters it that she WANTED to buy it, she had made a MENTAL COMMITMENT to by a TYPE of the cosmetic product that WOULD give her that particular look.
According to its role, the subjunctive is used to project a phantom vision.

I don’t know how to explain it but this sentence is kind of in the past but at the same time it kind of refers to the future in the past.

Would you help me explain this feeling?

On the other hand,

Una chica compró sombra para los ojos que le daba un aspecto vampiresco.

The way I understand this sentence (I think it is pretty close to yours):

The urban legend says that once a girl (unidentifed person (new information) bought a can of the eye shadow that made her look like a vampire but when she applied it the second time, she started biting people.

FACT 1: she bought the product
FACT 2: product was asserted/understood to have been found on her face

I don’t feel anything that would imply a notion of future here.

[aleCcowaN]

I don't quite understand what you are talking about.

The periphrasis "ir a + verb" implies future

va a comprar = he/she/it/they is going to buy

Va a comprar un automóvil cuando termine de juntar dinero para el anticipo.

Hes's going to buy a car once he has gathered enough money for the downpayment.

But

va a comprar = he/she/it/they is going in order to buy

isn't a periphrasis

Fulanito piensa en dónde va a estacionarlo mientras va a comprar el auto.

Fulanito is thinking where is he going to park the car while he's going to the dealer to buy it.

To make the field even muddier

va a comprar

and

va a haber

are both the said periphrasis, but

va a comprar = he/she/it/they is going to buy

va a haber = there is/are going to be

Many people say "van a haber muchos problemas", but I think many will frown while hearing it .

Fue a comprar

can't be that periphrasis, but

iba a comprar

could be it.

Iba a comprar ese auto pero me arrepentí

I was going to buy that car but I changed my mind.

but it could be not

Mientras iba a comprar el auto lo asaltaron y le robaron el dinero para el anticipo.

While he was going to the dealer to buy that car he was mugged and lost the downpayment money

The moral here is that the fact a periphrastic structure exists doesn't prevent the auxiliar verb to be used independently

estoy parando el tránsito= I'm stopping the traffic

estoy parando en lo de mi primo= me hospedo en casa de mi primo= I'm staying at my cousin's place

estoy parando los acreedores= I am at the end of my rope (financially)

[Nfqufktc]

Thank you, aleCcowaN. Your clarifications are vastly appreciated.