Unfortunately some of your journeys are one of the most popular and expensive ones…

Check this helpful website for reservations : https://www.seat61.com/interrail-and-eurail-reservations.htm

You can enter any pass number on B-Europe for Eurostar (I1111111 works). Reservations are tied to a first and last name and that number doesn't appear anywhere. 4€ fee per order -> https://www.b-europe.com/EN/Booking/Pass#TravelWish

Paris - Barcelona : either direct TGV for 35€ or

- TGV to Montpellier/Perpignan (10€ limited fare, then 20€) and last bit on regional trains

- night train to the Spanish border and regional train the following morning (13€ ticket so not worth using a pass day)

Barcelona - Florence will be best split somewhere between Marseille and Nice yes. Either 25€ TGV reservation to Montpellier/Nice, regional trains or evening AVE to Marseille.

Reservations for this high-speed train run by RENFE are not available online at the moment... either wait to book at any Spanish counter (but it's risky in summer) or get advance tickets. A shame…

The rest of the trip will be easier but definitely book seats in advance.

Questions always welcome. This summer will be really busy so I'd recommend booking early (although most trains aren't available yet).

Thanks - helpful advice.
Eurostar reservations -I’ve  seen you suggest using 

 1111111 for the PCN before. 
Is there anyway to confirm that works as other places like the Interrail website and Seat 61 all seem to suggest you DO need to have already bought your pass. 

Only when booking reservations via Interrail you need to buy a pass first. But it's best to avoid booking reservations there because they have an extra €2 booking fee per person and train.

Eurostar and any TGV can be booked without extra booking fees on www.raileurope.com.