Here is the preparation for the clay test:

But let's cut the clay and get a clear view of the cross sections involved:

This could be remedied by shaving a bit off this corner, since the sleeve hub is fixed in axial position :

I like the way you used those button head screws on the release plate.  That provides a lot more clearance.

I'm a little concerned about the gap that I see between the flange on the sleeve hub and your outermost drive plate.  That drive plate should be contacting the aluminum flange, but the photo shows what appears to be a large gap.  Am I seeing things?  I see a gap in several of your photos.

The clutch plate stack height controls the elevation of the release plate.  What is your stack height?

The extra washer you intend to install should have no effect on the spline engagement between the pressure disk and the sleeve hub.  Likewise, reducing the height of the hard stops will also have no effect on spline engagement.  The spline engagement is controlled by the clutch plate stack height.

I'm not sure how you arrived at 2.5 mm travel.  I never got more than .070" empirically.  I measured it with a dial indicator and also feeler gages.  As I recall, those readings agreed with my calculated travel.  That data along with the methods I used are in my original post.  I assume you measured your pressure disk travel with a dial indicator.  That should have been accomplished with the clutch plates installed without the wave washer assembly.  So, you ended up with 2.1 mm (.084"), do I have that correct?

By any chance did you measure travel without the special washer installed?  If so, how much travel did you have without the washer?

It would help us all understand your problem if you were to tell us what your clutch plate stack height is.  It would also be helpful if you could check your outermost drive plate with a feeler gage and verify that it is in intimate contact with the aluminum flange on the sleeve hub.

I might add that on my first iteration of this hotrod clutch, I had a set of plates that were significantly thinner than what I am currently running.  Over time, as I developed more & more power, I switched plates to eliminate slippage.  The set of plates I am currently running should be very close to the same stack height I recorded for this post.  Although my spline engagement is minimal, it has performed flawlessly over many miles.

and the free length of the sleeve hub is 33.4mm
this should give me 33.4mm - 31.2mm = 2.2mm engagement.

As a remedy, I first skim the friction surface of the pressure disk down to flush with the splines, this brings engagement back up to 2.2mm
It also eases the compression of the clutch springs by 0.6mm and it brings the release plate forward by the same amount, but I don't think this will cause amy interference.

I don't think I have a problem anymore, the clutch seems solid and has the clearance needed, only one more thing:
The hole in the end of the gear box axle, where the clutch push piece enters, has an unacceptable surface finish. I think someone forgot to load the reamer in the tool changer bay, in the machine shop that day they turned this shaft, because it is really ugly and not at all suitable for an o-ring to mate with, and allow axial movement. The original o-ring was all chewed up, and I can hardly make a fresh ring engage with that hole.
It is 12.4mm in diametre, the bottom of the o-ring groove in the clutch push piece is 9.5mm. Since the o-ring is a 1.9mm cross section, the interference amounts to 9.5mm + 1.9mm +1.9mm = 13.3 - 12.4 = 0.9mm which is also too much.

I have ordered an adjustable reamer so that I can get the surface finish on par with what is to be expected.

Have you experienced a hole like this, Mike?