I'm concerned that I will not be able to purchase train tickets from Amsterdam Centraal Station to Antwerpen as the website (ns.nl) warns that "Not all tickets for international journeys can be bought in the Netherlands." Is this truly the case? Any advice will be most appreciated.
I'm not sure what that disclaimer means, but you will have no problems purchasing tickets between Centraal and Antwerpen on the day of your journey. In fact on this route using the regular inter-city trains, advance reservations aren't possible.
You can buy the tickets in advance for a discount at nshighspeed.nl. You can even print them out at home.