I have a girlfriend, and a pass-by problems

[CroMuslim]

Asselamu alejkum. My name is Dino and i am from Croatia. Praying, watching lectures, reading Qur'an in the bus or tram, preforming dhikr, doing as much good deeds i can. Going to gym every day, learning for school etc. Year and 10 months ago i met one girl and i liked her, back then i wasn't praying, nor reading Qur'an, nothing i mentioned above regarding Islam, but when i met her and we were together ( cutting the story because it is huge ) i started to change dramatically. She is christian and she doesn't believe in church, she believes in God but i convinced her that Jesus as is not God as he is presented due his powers. Anyway, Alhamdulillah we didn't do any zina because i knew this rule back when i was kid and our relation ship is good, few months ago we reduces 'kisses' to minimum and lets say that we only enjoy hugs now :statisfie. From my perspective we are 'normal' people because in our country we are surrounded by kuffars, zina, alcohol, smokings, clubs, music etc and we are going through that inshallah. Since i met her, i started losing 'friends' which were taking me to the clubs drinkings, looking girls etc and now i have zero friends, and i don't go out anymore, rather to watch some lectures then to spend time uselessly outside drinking coffee and listening to rubish. Ok now you met me and now let me get to the problem. Year and half ago her mother found about us and we had big fitna regarding us. Her mother hates muslims because they fire up her village back when she was young ( LOL, group of people is representing religion? ... ) and after few months she cooled down due my girl said to her that we are not together anymore and since then we are ok, we don't go out often etc. In the Qur'an it says that we should respect parents no matter why ( parafrazing ) but her mother does not know what is best for her, i am acting like her father and forcing her to study, learn language that parents should do that but okay.
Question is : How should she behave to her mother because she is so defensive and aggressive towards islam but to not disrespect her? I told my gf to tell mom that we are all going to die soon and that her mother should realize that and stop acting like a kid and probably she will that that story to mother when we are inshallah going to marry.

I am giving dawah to my gf so maybe she will accept islam inshallah and i am making dua every day for 5 times for 4 months already, or more i don't know.

Question 2: Is it age of 21 best time for marry? Imam in masjid where i go said so and we have in plan inshallah to do then ( we are 18 atm ), also i kinda have mastrubation problems also that is one of the reasons, i want that our relation ship is valid and that i don't have it always on my mind that i am making haram. ALSO i would feel more secure with her going through hardships etc.

I hope someone will help me and i will ask more questions as discussion goes.
Sorry for my bad english in some parts ( tenses ), i am not native english speaker and i am learning as much i can on every side ( Islam, school, english, arabic ).