Thank you all for your answers.
Regarding the screen protector: as I wrote, I too in the ereaders I have had never put it on, but in this case, given the cost of the Elipsa, I asked for your opinion.
After a few days of use I too believe that the screen protector is not needed: in fact, the screen seems to have been "treated" to make the pen slide better. Any film would change the smoothness of writing (I think almost certainly for the worse).
Regarding the cover, however, I am still very undecided.
In the meantime, I ordered
this one on Amazon (which I paid very little for, only 13.50€). The quality of the materials is decent and I think it is good that it wraps around the ereader, even the corners, protecting the whole ereader, not just the screen.
On the other hand, the build quality is not great: when closed, the cover does not adhere well to the screen (I have a similar cover on my Kobo Forma and it works much better).
But what I consider to be the real weak point of this cover is its weight, which is a whopping 267g! The total weight of the Elipsa with the cover and stylus is 670g: for long reading sessions this is a considerable weight.
To summarise, although not excellent, the cover seems to me to protect the eredaer well, but makes the user experience considerably worse.
I am really very undecided whether to keep it or return it to Amazon and find another one.
The question would remain as in the first post: which one to choose?
The original Kobo sleepcover, besides costing way too much (in my opinion), only protects the screen, but perhaps doesn't weigh down the ereader too much.
On Amazon, apart from the Kobo one and the one I already bought, there seems to be no alternative (the other covers all look the same as the one I already have).
Do you have any other suggestions?