Regarding your first point - keep in mind this is a pay to win game, those who pay will also have a big advantage. As a free to play player you should not focus on competing with the paying players, your focus should be on maximizing your rewards given your resources. If you actively try to compete with the paying players you will just make your playing experience miserable.
As for #2, getting event mythic girl as a free player is still possible, although it involves some item savings from doing multiple coloseum. you will not unlock the mythic girl in every new event, but it's possible every few events. with the older event being repeated it's possible to eventually collect all mythic cards if you plan on playing on a long term basis.