Using substantial parts of a paper you authored in your thesis IEEE is very slow to answer and I need some clarification rather soon. If I am writing a dissertation and want to use extensive parts (but not the full paper!) of one of my own papers that was previously published in an IEEE conference's proceedings, what do I need to state in the dissertation to satisfy the IEEE copyright regulations?

IEEE states that for thesis or dissertation reuse the following rules apply:


  Thesis / Dissertation Reuse
  
  The IEEE does not require individuals
  working on a thesis to obtain a formal reuse license, however, you may
  print out this statement to be used as a permission grant:  
  
  Requirements to be followed when using any portion (e.g., figure,
  graph, table, or textual material) of an IEEE copyrighted paper in a
  thesis:
  
  1) In the case of textual material (e.g., using short quotes
  or referring to the work within these papers) users must give full
  credit to the original source (author, paper, publication) followed by
  the IEEE copyright line © 2017 IEEE.
  
  2) In the case of illustrations
  or tabular material, we require that the copyright line © [Year of
  original publication] IEEE appear prominently with each reprinted
  figure and/or table.
  
  3) If a substantial portion of the original paper
  is to be used, and if you are not the senior author, also obtain the
  senior author's approval.   
  
  entire IEEE copyrighted paper in a thesis:   
  
  1) The following IEEE
  copyright/ credit notice should be placed prominently in the
  references: © [year of original publication] IEEE. Reprinted, with
  permission, from [author names, paper title, IEEE publication title,
  and month/year of publication] 
  
  2) Only the accepted version of an IEEE
  copyrighted paper can be used when posting the paper or your thesis
  on-line. 
  
  3) In placing the thesis on the author's university website,
  please display the following message in a prominent place on the
  website: In reference to IEEE copyrighted material which is used with
  permission in this thesis, the IEEE does not endorse any of
  [university/educational entity's name goes here]'s products or
  services. Internal or personal use of this material is permitted. If
  interested in reprinting/republishing IEEE copyrighted material for
  advertising or promotional purposes or for creating new collective
  works for resale or redistribution, please go to
  http://www.ieee.org/publications_standards/publications/rights/rights_link.html
  to learn how to obtain a License from RightsLink.   
  
  If applicable,
  University Microfilms and/or ProQuest Library, or the Archives of Canada may supply single copies of the dissertation.


Am I correct in the assumption that for my case only the first case (marked bold) is relevant and I hence only need to a) cite the source and b) put a mark "(c) 2017 IEEE"? 

Does anyone have any hands-on experience with such a case?
The statement seems pretty clear and I'd suggest you are correct that the first paragraph applies. However, I'd also suggest adding the permissions paragraph (your bold) to the thesis itself as a foot/end note just to be clear to readers. Self-plagiarism is actually a thing these days.

I don't have actual experience with this, but can suggest a strategy. It seems conservative enough to satisfy IEEE given the above. 

Include your work as you like. However, don't just incorporate it, but actually quote the relevant parts of your papers, listing yourself (and others as appropriate) as author(s). This avoids the "self plagiarism" issue. You are treating your work just as you would that of another. 

You are also listing IEEE as the "publisher" of record by referencing it, so their interest in your work is preserved. 

See if there is some way to send a late draft of your dissertation to IEEE along with pointers to the referenced material, asking them, again, whether they have any objections. They may reply or not. If they do, you will have clear guidelines. I doubt that you would need to do extensive revisions even in the worst case.