Trains in Norway are covered by Interrail indeed. The ferry to Lofoten is not.
Hey Alessandro,
(1) depends on your plans and also on cities… if you tell about the exact route, will be easier to tell you where one day might be enough and where not 
in general I think for Lofoten after 5-7 days you will have seen enough. For the Lofoten you can use the “Billet Nordland” App. They offer a pass including also (some) ferries and the buses on the Lofoten. Price might sound expensive but for Norway quite ok.
(2) When I was in Norway trains were usually quite busy, especially between Oslo and Bergen, but also Oslo-Kristiansand. In the North I didn’t found them particularly busy. Only the Nighttrain Narvik-Stockholm is often busy. If you travel in July/August, then might be good, to buy well in advance! One more remark: also depends on how many people you are → the more people, the more difficult could be, to get reservations on a short note. I usually travel alone or with one other person → not really a problem to find a seat, but as a group… another story during high season!
(3) Yes.
Ciao, nice plan. Hope you have a ood time.
Just as the Frecce and IC of TrenItalia in some countries all such long-distance trains MUST also be reserved-find details on the general site, look for country info. What you pass: Sweden and Norway this must be done and can be made online, but every time you do it will cost a few EURo. For now also in Denmark-that is because of corona-distancing measures and it is unknown for how long this will be.
In Germany-the nice white ICE trains, you can reserve, but do not need to-but on peak days trains are often very full-and also much often delayed- so you may have to stand or cannot sit all together.
Up north in Scandinavia the main summer holidays are much earlier as in IT. They always start on 21/6=longest day (midnight sun over there!) and end by late july. That is peak-time for travels there.
Check also very carefully about boats/ferries in Norge-here is a general country wide timetable for all transportation: rutebok.no - let ITaly take a look and copy such a thing! Norge is very, very expensive, also for food.
You did not say where you line in Italia-but starting back from Narvik f.e.-and use the nighttrain to STOckholm (and visit the ABBA museum-your mom+dad can let you hear their music) and then again next night to Malmo-Copenhagen, reach south of Germany late that day 3 to then go to Italia on day 4. That is the quickest way.
Use bahn.de for best general travelplanner all over Europe. Often summer timings are not yet available-just use a same weekday in may or so to have a general check-then update later. It is usually possible to REServe seats 3 months advance in these countries, you do not yet have the pass then to do it.
