It looks unrealistic to be able to claim every month an amount of in excess of 10% of your gross salary, at least IMHO. That would mean one has to spent every month 1000-1500 Euros only on company expenditure, something that looks difficult to achieve
It is unrealistic, unless you are out of base - as you said. The big thing that most new-joiners live on for about 2 years is the cost of Type Rating and subsistence expenses from their time in training. When those expenses are gone it, reality catches up and people are surprised that they all of a sudden have a much lower retention rate. And to boost their take-home pay some start to put in dodgy expenses, like driving there car half way across Europe on every block off. Of course they don't, but since for mileage no receipt is required it goes through. At least until IRS knocks on your door.
Furthermore, I was told that if you are in a base where there is not much flying in winter time, then obviously you are looking at a salary of 30-40 hours gross, making the home pay on such months, very very low. Is that also true?
It's true. When a base reduces from 7 aircraft in summer to 2 in winter - it's easier to understand. Now, everyone's month off is distributed during the winter so that offsets the reduction in aircraft partially. This seem to be more of a problem for FOs though, since captains tend to be closer to the 900 hrs limit over the year.