I watched the Vocal Workshop DVD in the German Recording Magazin 4/2017 and was amazed at the smooth workflow they had.
They are using a Mac so I tried reproduce this on Sonar. The problems I have are:
 
1. After I flatten my comp in Sonar, all clip boundaries disappear. How do I prevent this?
In the demo (on a Mac) the borders do NOT disappear, allowing further correction if necessary.
 
2. When I open my flattened "Comp" track in Melodyne,
a. The contents of my "Comp" track disappear, and
b. when I hit Play, the Now cursor moves but I do not hear anything.
But when I move notes in Melodyne, I DO hear it!
I can unmute a take and then I hear that, but I do not hear Melodyne (and the Comp track is gone)
 
Any advice?
 
Win 10, SPlat 2017/5, Cakewalk UA-25 ex.

I would recommend always copying your flattened comp to a new track before editing or using region FX, otherwise, you can get problems. So unlock the flattened clip (top take lane) and copy to a new track. You don't want to copy the whole comp with take lanes, just the one flattened clip.

THE FORUM WIPED OUT MY RESPONSE WHEN I EDITED IT!

Attempt 2:
 
Your suggestion works great, thanks!
Seems I do not even need to flatten the comp. I can just drag copy the top track into a fresh audio track.
 
But in future I'll do it without comping tools.
1. Set Preferences/Editing/Clips/Selection after single split: Right portion
2. Swipe-Select all the Vox tracks in the Inspector
3. Split between the phrases by selecting the position in the timeline and pressing 'S'
4. Re-select all tracks and press 'K' to mute all clips
5. Moving from begininning to end, audition each clip by selecting and unmuting with 'K'
6. Copy all clip phrases I want to keep into a fresh audio track with Ctrl+Shift
 
A bit more work but no unexpected side effects so far.

What is a "flattened comp?" I assume comp is your final vocal take with all the best pieces, but what does it mean to "flatten" this?
 
Just curious.  Thanks.

konradh
What is a "flattened comp?" I assume comp is your final vocal take with all the best pieces, but what does it mean to "flatten" this?
 
Just curious.  Thanks.

It "bounces" (per say) all takes to the main active track

If you use the comping tools you have several tracks (takes) automatically named T1 to Tn.
 
You snip the takes into clip stacks. Per stack you activate one clip, the rest are muted.
 
When you are done, you right click to open a context menu with a menu point called "Flatten comp".
 
It automatically creates yet another take track and automatically names it "comp".
The menu point bounces the active clips down into this track.
 
The documentation calls this "Flattening a comp".