BJ - you are right - thanks. Great photos and illustrations at the link you provided.

Randy - thank you - good advice. Had not thought through the order of ops for mudding yet.
I have dry-fitted the 2" standpipe joining the p-trap & the Kerdi drain (KD). Once the concrete is dry, I will cut the standpipe to seat the KD at the right height off the slab, then glue the standpipe and KD in place before
mudding, etc.

By way of update -
Had the plumber in on Thursday. Poured the sub-floor this afternoon. The trap and 4" ID spacer pipe are in place. As pointed out by BJ, the KD is 4-1/2" OD for the first inch or so, so I cut the spacer off below the surface of the slab and used a bit of tin as a temp outer spacer until the concrete was firm enough to hold its shape.
The tin is now out, so lots of room for the KD.
If I ever do this again, I'll use a 5" ID spacer pipe.

Two issues I can see coming up:

1. How level is the slab and do I need to pour a bit of Self Levelling Cement (SLC in this forum) to make it "just so" for the shower pan ?
Think I will just place the Kerdi shower pan on the slab & measure for level. Assume mud can fill in very minor dips, but anything more will require SLC.

2. When I seat the standpipe and KD, do I need to fill the entire space between the 4" spacer pipe and the 2" standpipe with mortar ?
From reading the Schluter stuff, does not seem to be critical, but likely smarter to do so than not.

Next step - some framing and cement board mounting - then time to get mudding with the drain & shower pan.

Have photos - will send to Harry once I have a few more steps completed....

if you could send pictre then that would be great.

Photos sent to Harry - hope the file doesn't blow up his inbox....