Hello blueturtle191
1/ and 2/ You can buy day passes only if for a return trip (for example Milano - Lugano - Milano). If you go to the airport, you need two one way tickets (Milano - Lugano and Lugano - MXP).
3/ No, such tickets are not valid in the funicular.
4/ Tickets are also valid in the buses in Lugano, but only during the period of validity (4 hours).
5/ Such tickets must not be validated. Show them to the staff in Milano Centrale.
6/ You can reserve seats in the Eurocity trains, but note that Arcobaleno tickets are valid only in the regional trains in Italy. No seat reservations are available in regional trains.
7/ The seats are almost the same in 1st class.
8/ Toilets are available, but sometimes out of order…
Pleas note that not all cars of the trains continue to MPX, observe the indications to board the right ones.
Kind regards
Danhiel