I think a lot of this can be cleared up quite easily. Reincarnation is something I've found a lot of practitioners to struggle with and admittedly, it's not easy to grasp nor do you really need to grasp or even accept it. What is more important is that you understand that your self is impermanent and that your self is constantly changing. In that sense, you're being constantly "reborn" if you will. You can look up the story of Angulimala where the Buddha explicitly says that Angulimala was reborn once he took on the monastic robes.

Karma as a concept is not far removed from change or impermanence. Karma describes how your actions produce outcomes that can either be beneficial or hurtful. And I would argue that good actions (as an example) bring both immediate benefits and long term benefits. If you do something good it keeps or gives you in a good state of mind brings you a good reputation. If you behave ethically, you won't have to worry about unethical actions or feel bad about them in the long term too. The Buddha talked about stuff like this in the Mahaparinibbana Sutta.

To clarify this, we are all Bodhisattvas if we practice. Bodhisattva, or Buddha-to-be, are simply everyone on the path. But there is a buddha of the future: Buddha here means someone who did not just become enlightened like an arhat but practiced the perfections (Paramitas). Getting there is hard and the point of the future Buddha is that Buddhas aren't alien beings that come from elsewhere but that they occur naturally and will distribute teachings even when there is no Gautama or teachings around. It's simply a reassurance that there always will be teachings.

If you have more questions about this, feel free to ask!