You're too early. Operators in those countries have not published all their timetables yet. To get an idea, check timetables in February or March on the same day of the week. Use a reliable planner, such as the Deutsche Bahn website and certainly not the Interrail website or the rail planner app, which both are not reliable because they don't get updates frequently enough.

Here is some useful information from the experienced travellers in the Community regarding both planning, reservations and activation of pass and travel days. 

 Planning

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

 Reservations 

Lisbon to Madrid is possible but quite difficult. Seat61 explains how to do it: https://www.seat61.com/trains-and-routes/madrid-to-lisbon-by-train.htm

Barcelona to Nice is also a bit challenging. You’ll probably need a TGV to Narbonne, Montpellier or Nimes, then a TER or IC to Marseille and then a TER to Nice. Renfe might restart their AVE Barcelona-Marseille by the summer (currently suspended), but it’s still to be seen whether Interrail will still be accepted on it and even if it is, its timetable up to December was not suited for onward connections to Nice, and it isn’t expected to change.

Nice-Firenze requires minimally a change in Ventimiglia (on the French-Italian border) and in Milan. It is probably nicer - even if it might require more transfers - to follow the coast to Pisa (through Cinque terre) and from there to Florence.

The rest doesn’t seem to be very challenging: plenty of trains between Florence and Rome and between Rome and Naples. Simply look just a few months before departure and book the necessary reservations. For italian trains book through oebb.at, see the reservations guide in this community:

Barcelona to Marseilles is best approached by regional trains to Port Bou/Cerberes, then various SNCF combinations via Narbonne. Typically  takes about 6 to 7 hours. The alternative is to take one of the 2 BCN - Paris TGVs at an extortionate rate to Beziers and back to Marseilles.

The key station from Ventimiglia is Genoa, where you can continue to Milan or change for the coastal run to Pisa and local train to Florence.

 

It seems that the in DEC cancelled direct ESpanish RENFE-AVE hi-speed MAY come back from around Easter, or not daily, but this will cost you. As above-via regional trains can do ev 2 hrs and cost nothing extra to pay, so no worries.

You mention 6 trips and do not say if any sidetrips will be needed. There is no pass for 6- just for 5 or 7-so do the math-the shortest and cheapest sector is likely the last 2 in IT-IF you can commit yourself to a fixed advance booking. OR consider taking a direct Bus=coach for f.e. Lisb-Mad (can then also do overnite)

The difference in price between a 5 day and a 7 day pass is only 54 euro for 2 days of unlimited flexible train usage v long slow journeys in a bus or the risk of pre-buying a cheap ticket with no refund or exchange.

When you are spending many hundreds of euros on your holiday is a saving of a few euros really worth worrying about? (54 - costs of alternatives).

p.s. where have you heard about Renfe resuming an alternative AVE service to Marseilles? Or is that just another of your unsubstantiated stories?

That is actually true, see e.g.:
https://www.railtech.com/all/2023/01/11/renfe-plans-to-run-trains-in-france-before-summer-following-end-of-sncf-cooperation/

As ever a supported reply. Thank you, it is very informative to see the full story.