The rail planner app is only updated every so often, whereas operator sites are more dynamic. The app is ok for overviews of routings and reservation status but always plan using several planners.

Your pass is usable on almost all trains in Europe.

There are lots of trains from Wien to Budapest. You’ve probably put in Bucharest by mistake!

You can only get to Sofia via Romania. The choices are Bucharest to Sofia via Ruse or Craiova to Sofia via Vidin. There are no other rail options - you can carry on to Istanbul from Sofia on the overnight train but you have to come back the same way. 

I’ve just come back from a few weeks in Eastern Europe so a few tips. Expect trains to be late the vast majority are (occasionally very late 2hrs +). Do not rely on tight connections except the cross border connections which are normally held .Always have plenty of local currency in case you need a hotel or taxi transfer. Always agree the price of a taxi journey before putting your luggage in the car. Check train times on the national carriers websites do not use the wall mounted station departure boards. Never use the JourneyPlanner app. It will be an adventure!! Most young people speak some English and the people will try to help you if they can. 

A quick look at a railway map will show there are very few routes to the south of the Budapest/Vienna line. There is a route from Budapest to Ljubiana then on to Trieste. Anything below that is reputedly disjointed and of variable reliability. They will all inevitably focus on Ljubiana.

However once south of the alps there is a logical route across the top of Italy (Venice - Verona - Bologna - Milan - Genoa - Ventimiglia - Nice - Marseilles - Paris or Lille - UK.

Alternatively, if you do find a way to Greece you can get ferries across the Adriatic into Italy.

One word of advice - budget well. Trips like this can swallow your money very quickly.

You’ll find Romania and Bulgaria quite cheap. I’d recommend Plovdiv in Bulgaria as a fabulous place to stay.

Check seat61.com for best current overview of howtodo those trips (its based on giving overland routes from UK to all those countries). And as above; NO trains for now between the old parts now indeependent republics that one were Yugoslavija, thus much hindering your plans to come back on another route.