Welcome to Gurupaara! Congratulations for passing your O/L's well!
Though I did not follow the Arts stream, I could still answer the concerns you've mentioned above.
1) Here is what the A/L application for 2018 say:
Subjects have to be selected in accordance with of the Circular No. 2009/16 and the amended circular No. 2009/16 (i) and ED/01/12/2/06 to 01.06.2013 issued by the Ministry of Education.
And here's the circular mentioned above. http://www.moe.gov.lk/english/index.php?option=com_circular&view=circular&id=66&Itemid=920
If your selection of subjects don't fall into those streams they've mentioned, it'll fall into the "Common" category.
2) Depends on your definition of how a subject should be tough or easy. It's completely personal.
3) & 4) Z score depends on the mean performance of all who've performed, your raw marks & the standard deviation. Check this out. http://studentlanka.com/2014/07/26/how-z-score-is-calculated-for-university-selection-in-sri-lanka/
So what does this say? Your Z score completely depends on how good you are compared to the rest who've performed. Some say that selecting some rarely selected subjects can help you gain a good Z score, which is not systematically true but statistically correct. It's not hard to guess why It's the competition.
Knowing how the Z score is calculated is completely unnecessary. Be hundreds of steps ahead of your peer group in order to have a great Z score, in whichever stream you choose. Also follow what you love! Good luck for your A/Ls