Is it possible my partner could have become pregnant from pre-ejaculate/sperm residue seeped from underwear on our hands coming into contact w/ vag?

Yes. The highest concentration of sperm is in the first ejaculatory impulse and if sperm get to the vaginal area pregnancy is possible. I had a patient who had 3 children despite the fact that she could not be penetrated. Check a pregnancy test if her period is late and encourage her to start a reliable method of contraception. Sperm swim very well!