How to protect your authorship when trapped in "collaboration"?
how can I...keep the credit for my method if I disclose it in a paper with other collaborators? 


You can't: Credit will be shared between all collaborators.*


  Will I be able to also put these results in my Ph.D. thesis and safely
  be recognized as its single author?


Yes.

*In some sense, you can get credit, since the result appears in your thesis, which suggests that you are primarily responsible for results. But, your published paper is more likely to be considered the primary source of the results. (I suppose you could submit your thesis before publishing the paper, or you could release a preliminary draft of your thesis before publishing, then you could cite your thesis/draft thesis in the published paper, which may give you slightly more public credit.)


  should I disclose the new results already and publish those in the paper we are preparing or should I keep this and prepare a new one?


This seems likely to produce opinion-based answers, which are off-topic here.