yes you can take any free non reserved seat :) 

Reservations are optional on this train if you want a seat for sure you can reserve for 3€ on the Website of Austrian Rail or Czech Rail. 

NO, any NON-RES seat-as about all trains where RES is optional, there will be tiny screens above/on the seats that will show for what sectors that seat has been RES. So it well may be that the empty looking seat is RES from stop 1 or 2 onward (Breclav, Brno). This applies as such to about all trains in AT/DE/CZ/CH/DK/GB and in NL and BE seats cannot even be RES for inland only. It will thus also give you an easy chance to visit those towns en-route, like Brno-may not warrant a visit as such, but a 2 or 4-hr stopover is thus well possible to get a glimpse.

This for the 2-hourly RailJet trains by CD and OeBB. There are also a few RegioJet (note the tiny difference) yellow trains, as such they will also accept passes, but as they only do RES seats and have more restrictions and also not offer old-style 2nd/1st cl, visit their office-in both stations, if you plan to use that. IF this is the only trip to do on that day-and if you have passDAYs, then separate booking for them may well turn out to cost a lot less-more so if you happen to be student wit ISIC or senior>65, as then the CZ part will be heavily discounted. THese groups can also enjoy free or heavily discounted local use of tram-bus in Prahy.