Is what I do — fondling a girl who practices prostitution until ejaculation, without full penetration — considered zina (adultery/fornication)? And is it permissible for me to marry her a misyar marriage, despite my fear that she might continue her work, as she lives in another country?

It is forbidden for a Muslim man to marry an adulterous woman unless she repents sincerely and her rectification becomes evident, because she cannot be trusted with herself or with her husband's bed, and she may transmit deadly diseases. However, if she repents and covenants with you to repent, and moves in with you so that you may protect her, then there is no impediment to marrying her. But if you intend to keep her in her current state, then it is impermissible for you to marry her, and you must immediately sever your relationship with her because it is a forbidden relationship. What you mentioned about your relationship with her is fornication/adultery in the general sense, so you must repent to Allah.