I will try point you around the best I can, but without some tiny bit of knowledge in how games work exactly, this could get difficult.
[quote]- How to make doors and correspondent switches / levers? I would have also some hidden switches, maybe a puzzle where a switch opens one door but closes two at the same time. By door I mean an obstruction into which the avatar bumps into horizontally.[/quote]
There are many many things going on in this paragraph.
Firstly, for the [quote]How to make doors and correspondent switches / levers? I would have also some hidden switches, maybe a puzzle where a switch opens one door but closes two at the same time.[/quote], you will need to use event and array to get the job done. The array will keep the states of the door. You can use value like 0 or 1 to imply whether it's open or close in the array, and you can put the logic of these doors in the event.
Secondly, when your avatar bumps into an obstruction that you called "door" there, you can check for a sprite collision in your event sheet. There is a similar thing called "overlap", but let's not talk about it right now.
[quote]- How to create readable signs? Like in Lost Vikings: you approach a terminal / sign, press the "use" button and a text pops up.[/quote]
In C2, you can create Text object in your layout. In your event sheet, in EVERY TICK, you can check FOR EACH of these Text object whether the player object is near to these Text Object or not. If the player is in certain distance, make the Text Object visible, else, make it invisible.
[quote]- Collecting keys, optional treasures, I want to keep the player interested.[/quote]
Once again, in your event sheet, have your player Object check for collision against your shiny items objects. If collision is detected for a treasure, you could use a global variable for, for instance, keeping score.
To create a global variable, right click on an empty area of your event sheet, and you should see global variable listed as one of the right click option menu.
A global variable can be named anything, and could be used to store a number, text, etc. In this case, you can go for number. If the player collided with a golden coin, you can add some values to the value of the "score" global variable. Something along these lines.
[quote]- Building a set of levels instead of one long scrolling screen, with an exit and points of no return.[/quote]
For this, you can create more layouts. When you have your C2 project opened, on C2's usually right panel, the one with "Projects" text label on it, do you see something like a folder with the word "Layouts" next to it? Right click on it, and you should see "add layout" as one of the right click option.
To save a game, C2's newer version has these cool built-in save feature. Please read https://www.scirra.com/tutorials/526/how-to-make-savegames
I don't think you may be able to understand all of these in one go, but here are some general guidelines and after this, it shall be technical details. Post more in the forums one subject per one forum topic to get better further technical assistant. Everything you have requested so far could be done in C2 and it's not so difficult, once you get the hang of how C2 work. You can do this, my friend.