To answer your second question, your partner will be better to use a Eurail pass. The only significant  difference is the in/out rule, which does not apply to Eurail passes.

Seat reservations are optional but all the trains are likely to be very busy so I’d reserve. If your itinerary is all you’re doing it may be cheaper to buy ordinary tickets. You can check fares on ÖBB. As a basic rule if you hold a European passport it’s Interrail anything else is Eurail pass.

Yes for traveling in czech republic an austria, most trains have only optional reservations. If you want to reserve, do it directly via CD or ÖBB, both 3€/seat instead of 6+2€ of interrail.eu/eurail.com. 

Sawasdee khrap. Note that in that period some trains may not run, especially around phee khao/mai (old+new yr). ON the days just before/after these long weekends RES on IC-trains is recommended.

IF these are the only trips you intend to make in that period, start/finish in CZ, then a Global pass will not pay off. Just purchase advance tickets for the longer trips online. In that case use RegioJet yellow train for come back od Wien do Prah for lowest price.

Thank you very much for all of your answers, I will check the cost of the individual tickets again. Further more, like in my case, I hold Thai passport but I also have Czech resident permit. Which type I should go for? Is it interrail?

In that case, you qualify for both. Prices are the same but Eurail doesn't have the limitation of max. two travel days in your country of residence, so I'd go for that.

Oh, i see. Thank you very much