Is it permissible to marry a Muslim woman who became pregnant through fornication with a non-Muslim man, knowing that she has repented, and that the Muslim community is aware of her act of fornication? And would the husband be a forbidden (mahram) relative to her child resulting from fornication?

It is permissible to marry a repentant woman whose conduct has improved, but only after she delivers the child whose lineage is not attributed to you. You may be rewarded for your intention to protect her chastity, and all her children would be considered mahrams to you. If you fear difficulties due to her past, then seek someone else.