You cannot book those reservations online, only at ticket counters. The only exception is the Budapest 22:45 - Bucharest 15:05 which can be booked on tickets.oebb.at

Budapest - Bucharest can be booked in multiple countries : Germany, Austria, Switzerland,... but Bucharest - Istanbul only at Bucharest train station.

Availability is generally not an issue on the day or the day before and worst case you follow the couchette carriage from Bucharest to Dimitrovgrad on regular seating carriages and board the bigger Sofia - Istanbul night train there.

I actually want to travel via  Veliko  Tarnovo stay a night  before heading out to Istanbul.

Presumably the same as above applies , buy my ticket in Bucharest , will i still be able to use my interail pass?

Thanks for the answer

You'll most definitely be able to book it in Veliko Tarnovo but probably also in Bucharest yes.