1.) can be comfortable i recommend traveltimes of max 5-7 hrs 
anything more can be quite exhausting
2.) Almost all Nighttrains need a reservation even for Standard seats and ofcourse Couchette and Sleeper are more expensive (These reservation will be charged on top of the Pass)
3.) Offical you can o everything with the Railplannerapp of Eurail but my experience is to have the local railplanner apps on the phone as these get more updates as the railplanner of Eurail
4.) Doesn´t know what you are asking for a Nighttrain often consist of the parts a seating coach (often compartment layout), Couchette (4berth up to 6berth Compartment) and Sleeper from 1 berth up to Triple (3 berth) compartment. Few Nighttrains offer connected Sleeper compartments (means you can combine 2 Sleeper compartment to a big one)
It would also have been quite helpful to state around that time of year.
Besides above you may/will also have to deal with various anti-covid measures, these differ per country and can be changed in a minute. Esp. currently in DE and IT you need to have the QR code=being vaccinated or recently tested or recovered. In IT they call this "”green pass'’, but its the same.
There are also by now that really that many overnighttrains to use on this route, unless you make a detour via Wien/Vienna/Austria. There are several sites that explain it all re nighttrains (f.e. seat61, which overviews all routings from the UK). By NOW the usable overnighttrains on this route are in germany=DE simply normal day-time ICE trains: only seats.
It would also have been quite helpful to state around that time of year.
Besides above you may/will also have to deal with various anti-covid measures, these differ per country and can be changed in a minute. Esp. currently in DE and IT you need to have the QR code=being vaccinated or recently tested or recovered. In IT they call this "”green pass'’, but its the same.
There are also by now that really that many overnighttrains to use on this route, unless you make a detour via Wien/Vienna/Austria. There are several sites that explain it all re nighttrains (f.e. seat61, which overviews all routings from the UK). By NOW the usable overnighttrains on this route are in germany=DE simply normal day-time ICE trains: only seats.
they could catch the Nighttrains from Munich to Milan,Rome or Venice
Maybe combine (depending on the time travelling) the Malmö - Hamburg sleeper of SJ (starting next year) or the Snälltoget Stockholm - Malmö - Copenhagen - Hamburg - Berlin with the Nighttrain from Munich :)
