So, I've a different counterexample that skips keys: a fold doesn't see keys that have been put and have never been deleted.

    7> C9c.
    [[{set,{var,32},{call,bitcask_pulse,bc_open,[true]}},
      {set,{var,46},
           {call,bitcask_pulse,puts,[{var,32},{1,1},<<0,0,0>>]}},
      {set,{var,48},{call,bitcask_pulse,bc_close,[{var,32}]}},
      {set,{var,64},{call,bitcask_pulse,bc_open,[true]}},
      {set,{var,65},
           {call,bitcask_pulse,puts,[{var,64},{1,22},<<0>>]}},
      {set,{var,66},{call,bitcask_pulse,fork_merge,[{var,64}]}},
      {set,{var,81},{call,bitcask_pulse,incr_clock,[]}},
      {set,{var,82},{call,bitcask_pulse,bc_close,[{var,64}]}},
      {set,{var,85},{call,bitcask_pulse,bc_open,[true]}},
      {set,{var,87},{call,bitcask_pulse,fold,[{var,85}]}}],
     {86273,69841,50357},
     [{events,[]}]]

Here is an annotated timeline of the race:
    
    folding proc:                      merging proc:
    -------------------------------    ------------------------------------------
    
    %% Fold starts.
    {list_data_files,[1,2,3]}
    {subfold,processing_file_number,1}
    
                                      %% Merge starts
                                      {merge_files,input_list,[2]}
                                      {inner_merge_write,fresh,new_file_is,4}
                                      {merge_single_entry,<<"kk02">>,
                                       old_location,file,2,offset,19}
                                      {merge_single_entry,<<"kk02">>,
                                       not_out_of_date}
                                      {inner_merge_write,<<"kk02">>,before_write}
    
    {subfold,processing_file_number,2}
    
                                      {inner_merge_write,<<"kk02">>,
                                       new_location,file,4,offset,19,
                                       old_location,file,2,19}
                                      {inner_merge_write,<<"kk02">>,keydir_put,ok}
    
    %% The fold reaches the place in
    %% file #2 where key <<"kk02">> is stored.
    {fold,<<"kk02">>,keydir_get,not_exact,
     file_number,2,offset,19,
     keydir_location_is_now,file,4,offset,19
     fold_does_not_see_this_entry}
    
    {subfold,processing_file_number,3}
    
    %% key <<"kk02">> is not found in file #3, nor is it found in
    %% any other file that the fold can process (in this case, file 3 is
    %% the last file in the fold's list of files).

If the keydir is frozen, then the keydir update by the merge causes the keydir to freeze, and the folder sees <<"kk02">>'s entry in file # 2, the keydir's frozenness will allow the folder to see that the <<"kk02">>'s keydir entry is that same place in file # 2.

But I'm not seeing an easy way to fix this merge race without consuming more memory ... I need to sleep on this.