Thank you for your answers, first of all!
It is really nice to have a place where newcomers to the way get some practical advice by "veterans"
It is - indeed - a tricky question.
I had a look on the bags you proposed, they both look quite good but super thin...
The "snugbag" even has a mosquito net integrated; to repell also "big bugs" - might be usefull...
I guess, the best will be to simply go to some shop here in Brussels, to see what they propose (and at what price; to compare all of it and then to decide whether to buy here or to order by the net - I still have one month and a half...)
For the downs, I simply forget them; Laura, your bedbug report is alerting...
(I didn't realise its extent; on the german and french pages about the camino, they do not focus that much on it... or I didn't want to see it?!)
I will probably try the layers-method with light-weight sleeping bag, a silk liner and something warm on me.
And I thought I could possibly fill a water bottle (or a special trekking 'bottle' like platypus) with hot - of course not boiling water - to improvise a "trekking hot water bottle"
(I will test this these days here to see the result)
Oh yes and, as I read somewhere here, an emergency blanket to put between the bed and me (bed bug prevention)
I am looking forward to get started - well, my head is already halfway on the road, now my feet shall follow soon....
FatmaG