Upvote:1
You are asking several questions, I'll answer nr 1 and 2.
According to the NS service page, you can't use a credit card to buy a discount subscription, but you can phone them if you have a (foreign) bank account.
Betalen met een buitenlandse IBAN
Heb je een buitenlands bankrekeningnummer? Dan kun je telefonisch een abonnement afsluiten via de NS Klantenservice of bij een Tickets & Service-balie op het station.
Google translate:
Paying with a foreign IBAN
Do you have a foreign bank account number? Then you can take out a subscription by telephone via NS Customer Service or at a Tickets & Service desk at the station.
If ordering by phone doesn't work out, you will have to go to a station, which would most probably be Amsterdam CS for you. If you really are tight on money, you could consider buying just a ticket from Brussels to Breda, and buy the subscription there. I doubt that's worth the hassle, though.
Upvote:2
The main problem is that you have to have a personal OV card to get NS Flex on it. I helped a couple of Ukrainian refugees, and there were problems with using non-Dutch IBAN. However, not in all cases.