anladım..çok mersi.(maybe it needs merçi like in french)

 Normally compound words [written as one word] are exceptions to vowel harmony[non-harmonic].  "akşamüstü" is a compound word here.

akşam + üstü = [literally top of evening] that means "dusk" , the time when the sun descends. the time when it gets dark.

other examples of compoun words that breaks vowel harmony ;

bilgisayar --> computer

bilgi [data] + sayar [counts,counter] = computer

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hanımeli = name of the flower [honeysuckle]

hanım[lady] + eli [hand]

 "Mersi" is correct...

şakaydı... kusura bakma

  önemli değil..kusur yok ..anladım..

I think B. O şehire ne zamandır gitmiyordum.

Because when you add a suffix to şehir that starts with a vowel, şehir normally drops its last vowel (i).

Şehir + i (ablative suffix) = şehri

Şehir + in (genitive suffix) = şehrin

Şehir + e (dative suffix) = şehire

I assume this rule also applies for all dative suffixes added to ´apocopating´ nouns. Doğru mu?

Dün nehire bir adam düştü.  (A man fell into the river yesterday)

And I also notice that 2 verbs also ´drop a vowel´ when they become passive

çevirmek / çevrilmek (to be translated), 
devirmek / devrilmek (to be turned upside down)