Thanks so much  @MartinM :)

Does that mean I need to have two separate bookings/transactions for the Verona-Munich train?

choose the “one-way tickets and day tickets” first then make the payment of 50 EUR

after that restart the process and select “seat reservation only(no tickets)”, then make another payments for 6EUR, then would I be able to see the locations of the seats before the payments?

When I remove the 50EUR one-way journey from the basket, I clicked the service and price details, and I saw the location of the seats! 

Does that mean I can do it multiple times until I get my desired seats?

Yes, exactly. You make two bookings and then can pay it all by once (as it is shown on your screenshot).
If you click there on “Click here for service and price details, print timetable” you should be able to see the seats assigned to you (as I showed on the screenshot earlier). 
You can try multiple times, but you have to adapt the “preferences” for that. Because otherwise it will offer you usually (from my experience) the same seats as long as nobody else books them (I book regularly through ÖBB website nighttrains for my father and it is like that in that process).
 

Click on Reservation Details:

And then you will be able to choose your preferences:

If you do it all at once, you have to click on the trip “Verona-München” to see the reservations:

If you click on it, then you will see your seats:

Yes, exactly. You make two bookings and then can pay it all by once (as it is shown on your screenshot).
If you click there on “Click here for service and price details, print timetable” you should be able to see the seats assigned to you (as I showed on the screenshot earlier). 
You can try multiple times, but you have to adapt the “preferences” for that. Because otherwise it will offer you usually (from my experience) the same seats as long as nobody else books them (I book regularly through ÖBB website nighttrains for my father and it is like that in that process).
 

Click on Reservation Details:

And then you will be able to choose your preferences:

 

If you do it all at once, you have to click on the trip “Verona-München” to see the reservations:

If you click on it, then you will see your seats:

 

Thanks so much for another golden tip @MartinM :)

I will make sure to set the seat preferences first :) 

I actually had to remove the 50Eur from the basket and only leave the 6Eur reservation fee in the basket before I could proceed and see the assigned seats. 
When I clicked to see the service and price details while both transactions were in the basket, I couldn’t actually see the assigned seats. 
So would that be ok if I make two separate payments instead of paying all in one go? 

Sure that’s ok. You will so or so get to separate tickets… one time the supplement for EURAIL and one time the seat reservations. On the train you only need to show the supplement when the conductor checks the ticket. 

I could see it without removing the 50€ supplements. As written above, you have to click on "Click here for service and price details, print timetable" and then you see (depending on what you added first) the supplements and on the bottom you can see “Verona - München”. If you click on that, you can see which seats were assigned and book it all at once.
But as said it is also no problem if you book it separately, as the tickets/reservations so or so will be independent from each other.

@MartinM thanks so much, I think I got it now :) 

Just one more question, for Venezia Mestre to Verona leg, should I book reservation on ÖBB as well or on travel.b-Europe?

On ÖBB because you don’t pay any extra fees. 

Will do, thank you  

I am so sorry that I had trouble booking the reservations for Venezia Mestre to Munich on Oebb.at.

Followed by the guide from you guys, I clicked  “seat reservation (no tickets) and “one way tickets and day tickets” separately and added both of the costs into the basket. 

I tried multiple times in order to get my desired seats but unfortunately the system never assigned me the single lined window seats after I tried around 50 times, so I gave up and was going to just pick a not too bad option among the few options for the 6EUO seat reservations. However, just before I finalise the booking, I noticed there were already assigned seats included in the tickets with seat numbers, and in the seat reservations, I had another 2 assigned seats with different numbers. 

So my question is, do I still need to buy the 6EUR seat reservation? if I do, what would happen if there are difference seat number in one order?

Thank you in advance. 

of course you do NOT have to buy two times reservations .
It seems for first class they include the reservations for free - if you have second class, then it charges you 6 euro extra (but supplement is then only 10 instead of 15 euro).
DB and CD include also in second class supplements a free reservation, ÖBB doesn’t.

haha that makes sense now. So should I just buy the supplement for 50 EUR and no 6EUR seat reservation? 

Yes, skip the 6 EUR seat reservation. The 50 EUR is already quite expensive as it is.

Thanks @BrendanDB , will do :) 

Oh makes sense, from last timetable change you get a seat reservation for free via ÖBB if you book 1st class. Same happens for Interrail/Eurail fee for ECB.