The global pass is most interesting, since that wil include the journeys to and from France.
You first travel date will indeed be the start of the validity. There's no need to activate now; you just have to do that before you start your journey.
You can book trains whenever you want. Since 17 July is a Sunday, it's a good idea to do that in advance. The best place in Belgium is at a station that sells international reservations. That way, you don't pay Interrail's booking fees.
Merci pour votre réponse. Je ne comprends pas le passage sur les frais de réservation. Vous voulez dire que c’est mieux d’aller dans une gare internationale en Belgique pour réserver le billet pour sortir de Belgique en mentionnant que je voyage avec Interrail ? Je pensais réserver directement sur l’application.
If you book on the Interrail website (the app also sends you there), then you pay a 2€ booking fee per person and train, in addition to the reservation. If you buy reservations from the ticket office in Belgium or France, then you don't pay a booking fee.