You are correct, you do have to delete the RC1 MP before importing the released 5.5 MP (this is because we made some changes to Monitor types, so the RC will not upgrade to GA MP in-place)
And if you have 'dependencies' from nworks RC MP to other MP's - this is probably because you created some overrides, or some new Groups for nworks objects, something like that. These overrides or groups are stored in some other MP, and they are dependent on the nworks MP. This is why SCOM will not allow you to delete the RC MP.
Note - if you do create overrides, it is best practice to create your own 'overrides MP' for each MP that you customise. Don't use the Default MP for storing all overrides for all MPs.
Which other MPs are listed as having dependencies, when you try to delete nworks RC1 MP?
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