You're fine with the car hire if Indigo took your money and acted as a middle person, with them paying Europcar for you.
As far as TRAIN from Paris to Florence, I've done it overnight to make the time pass quicker but do remember arrival was pretty early in the morning in that case. I checked and for day travel it can take from 10-12 hours since you need to change trains, that's the part the causes variations. Overnight the trains take 12-15 hours, maybe on purpose to make departure and arrival times fit better to people sleeping.
As far as TRAIN from Paris to Florence, I've done it overnight to make the time pass quicker but do remember arrival was pretty early in the morning in that case. I checked and for day travel it can take from 10-12 hours since you need to change trains, that's the part the causes variations. Overnight the trains take 12-15 hours, maybe on purpose to make departure and arrival times fit better to people sleeping.