1., I think YES,as with any ticket you need this surcharge-BTW, its only 2€/ride and must be bought from the BLUE machines (=tickets for TER). I´ve done it once but there were no checks, so no hot experience from conductors (Duinkerken->Lille).

2.yes, the very first thing I´ve ever learnt about when computes came in this world: you can ONLY get out what someone has bothered to put in. There are no magic liiliputs that put everything they grasp in the big system, Some of the 100+ railways simply do not bother to put in all there offers and other only half or a few. Always-it has been told so many times, best to use planner of the railway concerned-and even then youĺl find that some are too lazy too bother much-or lack the money for proper paid IT-engineers. Next best is bahn.de, of german railways who do a quite good job in giving EUR-wide coverage. That of eurail is often outdated

1. Good question. For the TERGV trains that do appear, the rail planner app doesn't say a supplement is needed. The same in the DB planner - possibly because the SNCF doesn't include this in the timetable data they send to the European Timetable Centre. SNCF don't seem to be very clear on this supplement. They don't mention it in any of their timetables, such as their online planners and not even in their PDF timetables (there are lots of TERGV between Lille and Dunkerque). They're basically keeping the supplement a secret. @Nanja, do you have any more information?
2. SNCF haven't (yet?) sent their complete timetable to the European Timetable Centre. The DB planner doesn't know this train (and others, I noticed) either.

I have passed it on to the relevant team to have it checked. Cheers,