If you swallow semen after a blow job and swish water in your mouth afterwards, will there be any sperm left in your mouth? if so, will there be enough left to get a girl pregnant if she fingers herself with her saliva 10-20 min later?

Unlikely: That hypothetical scenario alone should not lead to pregnancy. In general, if a woman is able to get pregnant with fresh sperm straight from a penis, then she is also able to get pregnant with fresh sperm from a finger. However, the penis can put a much larger amount of semen right at the cervix. The finger is better suited for other activities.