here are the answers...
1) there is a change during the last years ... the normal seated night-trains get less, the special comfort night trains become more.
Normal seats in normal seated night trains mostly do not need a reservation. During the nights of a weekend, they are often full, especially in the summer months and in South European countries.
2) for night trains offered by the German rail company have a look at rail.shop/bahn CityNightLine
the information about the supplements I have right now (just changed for 200
normal seat: 4 EUR
sleeping seat: 10 EUR
couchette (6 beds in a compartment): 20 EUR
you can buy this reservation at German train stations, but it seems that the places for InterRailers are limited...
3) in an ICE a reservation is possible but not compulsory. if you want a reservation (good on Friday and Sunday evening travels) it is 3,50 EUR.
TGV - you need a reservation: 3 EUR for off peak times and 15 EUR for peak price times (ask at the station). normally peak times are in the early morning, in the evening during the week and on Friday and Sunday evening.
4) see above ...
Have a lot of fun,