Kenny202,makes some important points--I am not sure why the majority here feel you (or all Farangs) should just give in these situations. I have seen people walk away from their life saving , saying I am a farang so I will loose. The Thai legal system works on trying to find common ground amongst litigants (a compromise) very few reach the court stage. The first stage of compromise is at the police station.
Questions to the girl--could you show us where you withdrew from your Bank or borrowed the money from to buy all these bikes? etc will be asked.
In front of Authority Thai's do not lie well. If a compromise cannot be reached there (I am sure it will be in your case) then goes to an adjudicator (I have been through all this a few times & watched friends cases also) you can call witnesses in for this --like the place where you brought the bikes/people who know of the situation, I would be fairly certain she would call no one in, because, expecting a "Lower Level" Thai to stand up in that situation and lie for a friend over this--no.
The adjudicator then makes a Non Binding decision on who is in the right, he then usually gives a certain amount of days for you both to meet and make a comprise. If still no compromise then it goes to civil court--this happens very few times- but with his decisions already stated. The onus is really on the other person to agree.
On the situation of you "Helping" out around the work place.... yes you shouldn't do that, But you didn't fall time work-just helped out. The whole idea was that you would purchase the bikes etc in her name, & she would run the place for a % of the profits until you both got married.
However you kept seeing discrepancies in the money being taken which led to arguments and her leaving and now using these threat about ownership.