1) I have been chatting and talking to non mahram all these years until I realised how big sin it is.
Therefore I have stopped chatting, talking, seeing non mahram now. However, my last connection was a young Atheist girl until reality came across me that I'm sinning by talking with non mahram. Now she actually converted to Islam not because of me but because she got influenced by The Quran and Prophet Muhammad (saww). The scenario now is that she has accepted Islam and is yet to tell this to her family and friends and everyone else and so far I'm the only person with whom she can talk to about Islam. We don't chat about anything else except Islam, it's teachings, duas and that too just 2-3 times a day, so my question now is that is it permissible for us to talk or we should not be in contact anymore?

2) Is it a sin on her part if she's planning or thinking about marriage with me? Or she should avoid such thoughts and leave the matter on Allah's will?

1) Talking to her about the mentioned subjects is permissible.

2) Planning or thinking of a good thing like marriage is okey, provided that she do not get into not permissible talks.