Hey,

1. no, you can stay as long as you want in a city (but one travel day on interrail means that it is from 00:00 until 23:59 and there is a nighttrain rule - if you board a train after 7pm, you can stay on it until it arrives the next morning. This is way you can make the most of one travel day)
2. Yes, you have to add the trip for the return trip again. If you have a mobile pass, select the date and trains. If you have a paper pass, write down the trains in the paper pass or also do it in the app.
3. It is written in the Railplanner app. Not always 100% accurate, but mostly true. Which countries do you plan to travel in? Then open the country page on interrail’s website and you’ll see

Hello Tina

The IR global pass has no restrictions on when and where you use them, they are simply a pass to catch any valid train in 33 countries of your choice.

So there is no problems staying as long as you like in any town.

There are a couple of limitations, travel in your country of residence and some trains have a mandatory reservation requirement.

You can use any global pass in your home  country for 2 days without restriction, referred to as in and outbound days. These can be taken at any time and in any order  or even not at all.

Reservations are separate from your pass and there is plenty of advice on the community. They can be acquired from several providers and fees are often payable.

Here is some useful information from the experienced travellers in the Community regarding both planning, reservations and activation of pass and travel days. 

 Planning

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

 Reservations 

Thank you for your answers! 
 

I would therefore like to ask you:

1) so, if I leave Venice to get to Lisbon. For the return, is it sufficient that I indicate Lisbon - Venice as stages? Or do I have to mark every single stage I traveled on the first leg?

2) Is it possible to know in advance (before paying) which trains require reservations?

thank you all 

Interrail does not work on return journeys. It treats each as a separate journey. It can also work on individual train journeys or combination journeys.

When planning your trip (i.e. the full diary of your pass journeys) your intended journeys are recorded, but that is all. You need to add each train journeys to your daily pass, ideally train by train just before you get on the train. If you add all trains in one go then you have to delete if you have to change your plans (missed connections, cancelled trains or simply fancy an unplanned diversion.

Your planned journeys are not tablets of stone - you are in control.

If you check via “find train times” on the IR site for Venice to Lisbon you will find it is one of the most difficult train journeys in Europe.

 There are many changes, takes at least 3 days and can cost upwards of 100 euros pp in reservations. Also many of the Interrail routings take longer routes to keep you on the fastest trains.

Italy to French border (Ventimiglia/Nice) is relatively easy in a day and simple to reserve seats as needed. French border to Spain (Barcelona then Madrid) is a challenge for both reservations and timings (14 hrs to 21hrs) - expensive if you cross the border on a SNCF/TGV or you would need to use regional trains with no reservations. Finally Madrid to Lisbon is tortuous and lengthy - there are no direct trains.

That night train rule was changed a few years ago. What counts now is the date (not the time) when you board a train:

https://www.interrail.eu/en/support/interested-in-interrailing/what-is-a-travel-day