We're very glad seeing you try multiple approaches and have reached certain success.
To be realistic, allow me to say that this kind of issue (nuclei clumped together in histology slide) is near the limiting edge of image segmentation. If it's even difficult for human eyes to decide the separation, it's not so fair to ask a software to perform beyond that. If later we're aware of a better solution, we'll gladly update to you and the community.
As for your particular image, I guess we can only hope to tune the segmentation to reach an "acceptable" detection, and/or include per-image quantification instead of focusing too much on cell-by-cell level.