the smart ticket only present in the station.
That means you need to buy the ticket from the any train station at Italy ot France.

yes, you should buy this type of ticket before 14 days before departure

just mention you something
don't buy the EuroNight in smart ticket
it was a horrible experience for me last year

the night train will drive out from the station before depart
the train will expose under the sun
normally, the night train will depart after 8pm
the train will become extreme hot
then you need to sit in a seat without air conditionor
it is because the power will not be supply until 10-15 mins to depart
you can imagine the feeling of sweat trap in your cloth and after it dry

So I highly recommend you son't take the night train especially in summer

there are trains from Paris to Madrid

10:10  Paris(montpamasse) > 15:40 Irun , 17:05 Irun > 22:44 Madrid(Chamartin)
15:50  Paris(montpamasse) > 21:25 Irun , 22:15 Irun > 07:35 Madrid(Chamartin) (Night Train)
19:43  Paris(montpamasse) >  09:13 Madrid(Chamartin) (Night Train)

P.S. The Departure of the train may be delay
Don't make your timetable being tight

Night train is good option for this
but don't buy the seats
it will always disturb your sleeping ( light and comfort of seat)
Sleeping car(+50%) and Couchette (15-20%)is better for night train

europass is not suitable for you travel inside the country
you can buy your ticket directly in the station
it is less expensive than europass
For you want to travel country to country
Europass may be cheaper (but not too much)

I had bought the europass last year
So, I have the timetable of the train

I think you can't buy the ticket in London
but you can try

www.renfe.es <-- spain
www.sncf.com/en_EN/flash <--france