Can one atone for fondling and embracing a married non-Muslim woman, and does this fall under the threat of not entering Paradise except with her husband's forgiveness?

The discussion concerns the inviolability of the wives of mujahidin (those striving in the path of Allah) for those who remain behind, akin to the inviolability of their own mothers. It is also mentioned that whoever betrays a mujahid concerning his family, the mujahid will take from his good deeds on the Day of Resurrection whatever he wishes.

Repentance from zina (fornication/adultery) does not require informing the spouse. The atonement for what you have done is sincere repentance (tawbah nasuh): ceasing the sin, regretting it, resolving never to return to it, and increasing good deeds. You must conceal your sin and expedite marriage if you are not married. Otherwise, you must fast, guard your hearing and sight, and associate with righteous companions.