Timisoara-Stamora Moravica-Timisoara (5,65 RON * 2 = 11,30 RON = approx. 2,50€) [full fare+advance purchase discount by CFR]
Will you be able to purchase this ticket in advance? Because it doesn't look like you can buy such tickets via CFR website, so it's Romanian ticket counter only. While if you purchase it before departure, then it seems like maximum you get save is 10%
Vrsac-Belgrad-Vrsac 512,- RSD = approx. 4,30€ [round trip fare, source: http://serbianrailways.com/active/en/home/glavna_navigacija/putnicki_saobracaj/unutrasnji_saobracaj/cene_prevoza.html ]
Vršac-Belgrade is <100 km, thus it means that ZS return ticket will be valid for only one day. Are you intending to make a trip to Belgrade in one day? Because otherwise you will have to go with two one-way tickets or think about tricks of buying return ticket for route >101km (but then it makes sense to count whether it will be worth that ZS 20% return discount on domestic tickets)
What about the ticket Stamora Moravica-Vrsac? Could it be possible to buy it in Romania before? How much would it be? According to this source ( http://europetrainsguide.com/forum/index.php/topic,704.0.html ), 1,60€ * 2 = 3,20€ for both way (1 person)
It is possible to purchase such ticket in Romania, however your calculations have an error. Although rail distance between Stamora Moravița and Vršac is only 21 km, but there is border, thus according to rules ticket tariff will be calculated separately for every railway on the route of the ticket, which means that one-way Stamora Moravița to Vršac ticket should cost 3,2 euro=1,6 euro (CFR part, even if it's just 3 km from border point) + 1,6 (ZS part, 18 km)
Thus, the return trip Timisoara-Belgrad (1 person) would be 2,50€ + 4,30€ + 3,20€ = 10€. Right?
Nope, see above remarks. Perhaps you can save some cents (compared to "Banat Special") if you go with offer of the same nature - Vršac Special (Timisoara to Vrsac for 4 euro one-way or 8 euro return) + ZS domestic ticket
Last thing: are there reduced fares for a foreign 17-yo passenger with ZS and CFR?
Well, in Serbia standard discount is given only to children/youth <14 years old, while for other categories you can get special
SRB Plus Card (e.g. for ISIC/EURO26 holders it should cost only 100 RSD) and it will allow to receive 30% discount. In Romania 50% discount is given to pupils/students, but it's only for those studying in Romania. Also "youth" (<18 years old) passengers can get a discount in case they travel in minigroups (2-5 passengers) - two "youth" passengers count as one adult, but unfortunately for your case CFR notes that with odd number of teenagers one “youth” passenger is simply considered as ”adult” in terms of tariff
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