The most crucial journeys are the Eurostar and maybe Venice-Paris.  The pass holder seats on many departures on the Eurostar in July are already sold out.

Here you can check availability of pass holder seats on the Eurostar and also make the reservation.

https://www.b-europe.com/EN/Booking/Pass#TravelWish

You need a Pass Cover Number (PCN) and I'll put a link to the PCN generator below 

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

Yes, you´re dreaming-or living on the last century-as between the split-off new republics of what once was Yugoslavija NO trains at all over border. Which means using pass there is a waste of money and will only lead to frustrations. GR is also isolated by train from its neighbours. IST only has in summer season an overnight train to Sofia-which 1 car shunted off and sent to Bucuresti.

Dedicated railfans know some ways of using train to last stop-then wlak/bus/taxi it over border and find an obscure and probably cancelled (train broke) connection 20-30 kms ahead-normal tourists will not be very much amused by this way.

IN case you wonder: locals use their 2nd hand cars or the BUS. Which tends to be much more comfy as most of the old clapped out trains one sees there so often.

Only from Ljub (actually Zagreb) is it possible to rely again on decent trains. Or replan. Even when trains were running doing this trip in 15 days would mean big rush.