После её признания всё расследование стало понятнее, и развязка уже не казалась странной.

Because после requires the genitive case.

• Dictionary form: признание
• Genitive singular: признания

So после её признания means after her confession / after her admission.

A useful rule:

• после + genitive = after

Признание can mean confession, admission, or acknowledgment, depending on context.

In this sentence, because we have расследование (investigation), the most natural meaning is confession. It suggests that once she admitted something, the case started making more sense.

Yes. Её and ее are the same word in standard Russian spelling practice; the version with ё simply shows the pronunciation more clearly.

Here её means her:

• её признание = her confession

In many printed texts, Russian often omits the dots and writes ее, but learners usually benefit from seeing ё whenever possible.

Here it means the whole investigation.

This всё is the neuter singular form of весь (whole, entire) and agrees with расследование, which is neuter singular.

So:

• всё расследование = the entire investigation / the whole investigation

It does not mean everything in this sentence.

Because the subject is расследование, and расследование is neuter singular.

In the past tense, Russian verbs agree with the subject in gender and number:

• masculine: стал
• feminine: стала
• neuter: стало
• plural: стали

So:

• расследование стало = the investigation became

Понятнее is the comparative form of понятный / понятно, meaning clearer or more understandable.

So:

• стало понятнее = became clearer
• literally: became more understandable

This is a very natural Russian pattern:

• стало лучше = became better
• стало легче = became easier
• стало понятнее = became clearer

You could also say стало более понятным, but стало понятнее sounds more natural and compact.

Here развязка means the resolution, denouement, or outcome of the story/case.

In a mystery or investigation context, it is the part where everything comes together and is explained. So it is close to:

• the ending
• the resolution
• the final explanation

It does not literally mean a physical untying here, even though the word historically comes from that idea.

Because after казаться (to seem), Russian usually puts the description in the instrumental case.

So:

• странная = nominative
• странной = instrumental feminine singular

Since развязка is feminine singular, the correct form is:

• развязка казалась странной = the resolution seemed strange

This is a very common pattern:

• он казался уставшим = he seemed tired
• она казалась счастливой = she seemed happy
• это казалось невозможным = it seemed impossible

Because the subject is развязка, and развязка is a feminine singular noun.

Past tense verbs in Russian agree with the subject:

• развязка казалась
• not казался
• not казалось

So the feminine ending -ась matches развязка.

Word-for-word, уже is already, and не is not, but together уже не very often means:

• no longer
• not anymore

So:

• уже не казалась странной = no longer seemed strange

This is a very common Russian pattern:

• я уже не помню = I don’t remember anymore
• он уже не работает здесь = he no longer works here

Казалась is imperfective, while показалась is perfective.

Here казалась works well because the sentence describes a general impression after the confession: the resolution was no longer seeming strange. It presents the impression as a state or ongoing perception.

If you used показалась, it would sound more like a single momentary impression:

• тогда развязка показалась странной = then the resolution seemed strange (at that moment)

So in this sentence, казалась is the more natural choice.