I married a woman and I was in Khulwah (being alone with a member of the opposite sex) with her but I did not have sexual intercourse with her because she was too young, then I divorced her. Am I considered a Mahram (unmarriageable relative) to her daughters from her next husband?
I proposed to another woman and concluded the marriage contract but I did not have sexual intercourse with her then I divorced her. Am I considered a Mahram to her mother or not? Could you kindly advise?
As for the woman whom you concluded the marriage with and stayed with her alone but did not have sexual intercourse with her, her daughters will not be forbidden to you because of Allah's Saying when He mentioned the women who are unlawful for marriage:
...your step daughters under your guardianship, born of your wives to whom you have gone in - but there is no sin on you if you have not gone in them (to marry their daughters)
(Part No. 17; Page No. 370) The meaning of "gone in" in the Ayah is: Having sexual intercourse. When you conclude a marriage contract with a woman, her mother and grandmothers will be prohibited as soon as you conclude the contract even if you did not have sexual intercourse with her because of Allah's saying in the mentioned Ayah:
...your wives’ mothers
Allah does not stipulate having sexual intercourse with her. Allah knows best. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.