Could I get pregnant from fingering? What if there were traces of semen left over if he touched himself earlier? His hands were dry and not dirty

Very unlikely. Its certainly possible to get pregnant if semen were on his hand with fingering, but highly unlikely. Use contraception to have the highest chance of avoiding pregnancy.
No. Fertility success is NOT linear with semen count. So.... most men have 2x to 5x higher amount per entire ejaculate than needed so a small decrease from pulling out after ejaculating won't help at all. On the other hand, having a somewhat low count or not having an intravaginal ejaculation REALLY REALLY lowers fertility to near zero.