1) When Carlos finishes his range, do you delete the old sieve thread?

For right now no, but may be in the future.

2) Is it possible by sieving to find factors of some of the combinations already in the prp-files available?

Yes, but it is extremely rare that you will find a factor in the time it takes to find a factor using the sieve in general.

3) Can somebody please explain to me what "Nash weight" is?

It is the number of candidates left after sieveing from 1-256 ranges n=100,000 to n=110,000. (approximately)

The exact defination is available here.

http://pages.prodigy.net/chris_nash/psieve.html
Citrix