Any idea where to break the ticket to get a cheaper fare?
Well, it is kind of conditional and really depends on your exact travel plans, but in case you won't make stops in France after Lille, then you can go with Amsterdam-Lille (40,6 euro with RP) and for the remaining leg just get in advance discounted ticket for TGV from Lille to Paris via French Railways website. It will definitely save you time (TGV three time faster than going with regional trains and changes) and also a bit of money, but tie you to the specific train
The main question is, as far as I know NRT tickets in France are accepted only on unreserved trains, which means pretty much only TER. I discovered that between Amiens and Paris Nord there are so called Intercités sans reservation obligatoire. Are NRT tickets accepted on these trains?
SCIC-NRT should work, these InterCites doesn't require seat reservation and standard fare is exactly the same as for TER trains, so can't see why it will be impossible to travel with SCIC-NRT tickets (but just in case we wrote to SNCF and asked about it, now waiting for an answer)