you can try if ktchiu's way works. it might help in some cases, but the glue might not work always. you might get irritated in the process of extracting the screws but this will result in even more damage to the device. now, there is practically no solution to this problem without spending any money. you will have to get some tool extractor type of thing which won't be very expensive. I am just giving you the links for such products.

you have mentioned that "
I've removed the remaining 4 screws (with great difficulty, they seem cross threaded). Detailing the screws shows that the thread is damaged". the threads are still usable enough. if you find that either of the old screws are now damaged, don't use them, get new ones instead.

to remove those two screws, I'll quickly find some product for you. with great patience, you can finally extract them out also and can then use proper screws where needed for the entire phone.

I have some friends who work for Nokia Care and deal with such situations often. I'll ask them and report back in the next 12 hours. till then I'll find out some product which can be bought in UK