1. What are the rules about mixed connections from Nightjet delays with (global) Interrail passes? How common are heavy delays on the Amsterdam – Vienna route? For the 07:44 arrival at Linz Hbf from Amsterdam Centraal, if I missed the 07:55 and 08:55 (change at Selzthal) to Graz, would I be able to travel via Vienna without needing an extra seat reservation?
NJ doesn't have mandatory reservations in Austria and it's unlikely someone actually booked a seat Linz-Vienna, so by all means, stay in the train if your next connection via Vienna is faster.
2. Is there any food worth getting in Slovenia, Austria or the Netherlands which can't be bought in the UK and which wouldn't go off in a week (and, of course, which could be taken through customs)?
Stroopwafels. Available in any Dutch supermarket.
4. For the IC direct supplement between Amsterdam and Rotterdam, is there any price difference between tapping in on the platform and paying on the NS app?
If you have an Interrail pass, you don't need any supplement because Interrail is an international ticket. The Rail Planner App is incorrect.
5. I've heard of issues paying with bank cards in Albert Heijn; do any other chains in the Netherlands (or NS) have similar issues, and should either a Visa debit or Santander credit card be fine?
Credit cards are not accepted in all of Albert Heijn's shops:
In onze winkels kun je op verschillende manieren afrekenen. Zo kun je gebruik maken van zelfscannen en afrekenen met je mobiele telefoon, of gewoon bij de kassa en pinnen of contant betalen.
www.ah.nl
(Credit cards and foreign pre-paid bank cards are accepted in the shops at Schiphol, in many station shops and 2 specific AH supermarkets in the centre of Amsterdam.)
Other chains may have other policies. Credit cards in general are not accepted everywhere in the Netherlands. Maestro and V-Pay should be no problem. In addition, now that Maestro is being phased out, Debit Mastercard and Visa Debit should finally be accepted everywhere, since that will be the replacement. I'm not sure how far they have already been rolled out.