Lulilu, please don't forget the second website I mentioned in your welcome thread:
I would suggest trying that first. There is video that you can watch. The nail caps are not a good idea and are really only a last resort, for the reasons you mention.
Please do not, under any circumstances, permit any vet to pursuade you have declawing carried out. This is still legal in much of the USA but is banned in many countries on animal welfare grounds.
If you find that the Feliway does not work, then you could try putting some cat repellent spray on your furniture:
In your welcome thread, you said that your sofa was in a very poor state and might need replacing. In that case, I would suggest that you invest in a set on zip-on protective covers for the new sofa. Good luck! Please come back and tell us how you get on.