In order to travel from Wien to Salzburg with a help of 29 euro international (Spar type) discounted ticket you can use OEBB online ticket option for Wien-Munich or Wien-Zurich route (
https://ticket.oebb.at/bin/frame_ticket.pl?ticket=jticket.pl?ABBFSEL=8100003&ANBFSEL=8000261&LANG=EN or/and
https://ticket.oebb.at/bin/frame_ticket.pl?ticket=jticket.pl?ABBFSEL=8100003&ANBFSEL=8503000&LANG=EN). For 12/22 there are still a number of cheap 29 euro tickets available.
Same thing applies for discounted Salzburg-Prague ticket - purchase you ticket in advance via OEBB website (either via online ticket option or simply email OEBB with Spar ticket request).
Other options for Salzburg-Prague route (in case you've got some time) - Bavaria-Bohemia ticket (check out Germany page) or Einfach-Raus-Ticket till Austria-Czech republic border + domestic Czech ticket.
Concerning 7 minute change in Linz: usually in case official OEBB website gives such type of connection it means everything should be fully ok (train to Prague even might wait for the one arriving from Salzburg), plus OIC 643 arrives to platform #6 at Linz Hbf, while IC 100 to Prague departs from neighboring platform #7. Such type of connection seems rather usual since till 12/13 there are direct train from Salzburg to Prague with similar schedule (from new timetable they just cut it and now it will start from Linz instead of Salzburg).