It is ok to use "dık " when we mention future actions. It doesn´t sound bad. 

For instance ;

Ödevimi bitirdikten sonra duş alacağım.

I will have a shower after I finish my homework.

However, we can drop the actions for some cases like in your example ;

instead of 

okuldan geldikten sonra --> After he comes from school.

we can just say ;

okuldan sonra

Because we know that people will know it is "after comes from school " when we say "okuldan sonra" [after school] 

other smilar cases ;

işten sonra [after work]  = After I leave [finish] work

Namazdan sonra  =  After the prayer. --> After the prayer is over.

Namazı kıldıktan sonra - After I finish performing  the prayer.

Namazdan geldikten sonra - After I come from prayer [Mosque]

So, it just makes your message more detailed, informative but at the same time people might find it unnecessary info cus the context already gives us that information.

It would have been  politer saying like this --->" Ooooo Abla Hanımlar sohbet odasına  teşrif etmişler ! "

Sorun yok --> No problem.

Don´t worry  ---> Endişelenme! Merak etme! Dert etme!

It also depends on  your level of  worry. 

I mean, if your worry makes you sad then you can also say " üzülme " .

If you want to be more formal & polite, you can even add plural marking "ler,lar" when u refer the 3th singular person. 

Kendileri iyi bir yüzücüdür, ayrıca güzel de tenis oynarlar.

,

" Var " and " Yok" can be used as a head of an adjective phrase. There are few fixed phrases like that ; 

Var gücüyle ---> With all [existing] power of oneself.

Var gücüyle arabayı itmeye başladı.  ---> He started to push the car with all his power.

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Yok  yere  ---> for no apparent reason, for no reason, without reason

Yok yere kendini üzme ---> Don´t be upset for no reason. [without reason]

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

Yok pahasına ----> Very cheap, for a song [phrasal], to give something away

Arabasını yok pahasına sattı. ---> He/She sold his/her car for a very little money.

                                                He/She sold  his/her car for a song. 

Posted twice

Var and Yok can take the suffix "-lık " , When this suffix comes to adjective type of words, it alters them into "abstract nouns "

Var + lık --> Varlık  --> Existence 

Yok + luk ---> Yokluk --->  Absence, Nonexistence

Good can exist without evil, whereas evil cannot exist without good.

Thomas Aquinas [ Philosopher and Theologian ] 

İyi , kötü olmadan var olabilir oysa ki kötü, iyi olmadan var olamaz.

Var olmak = to exist, to be, to survive 

Bosna´da halk var olmaya çalışıyor.

People in Bosnia are trying to survive. [ trying to exist ]

Türkiye Cumhuriyeti sonsuza dek var olacaktır.

Republic of Turkia will exist till eternity.

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As a noun it should be written as one word; 

Tanzanya´da halk varolma savaşı veriyor.

People in Tanzanya fıghting for their existence. [ to survive, to stay alive ]