a Ger disowns his pre-conversion family
AFAIK, not true (actually, it sounds ludicrous to me), although halachically, mid’oraisa they are not related.
As far as honoring parents, IIRC, they should be honored because of darchei shalom and chillul Hashem.
Concerning as marrying them, IIRC, prohibited mid’rabonon (which is still mei’ikor hadin).
Regarding physical contact/yichud, I believe Rav Leff’s reasoning would be that there is no concern of it leading to an aveirah. This is the reasoning that a person is allowed to have yichud with a parent, although the parent is most definitely an ervah. (This issue is also relevant to an adopted child, although I seem to recall that the issue there is more complex.)